But there's a catch. You have to read the book straight through, from front to back. If you try to skip around, or just read the parts you feel you need, you'll probably be frustrated, because — exaggerating, but only slightly — every part of the book is linked to every other part. Perhaps if you're already expert in Git and merely want a quick reminder about something, it would work, but in that case you're more likely to do a web search anyway. For the rest of us, taking the medicine straight and for the full course is the only way. To some degree, this may have been forced on the authors by Git's inherent complexity and the interdependency of its basic concepts, but it does make this book unusual among technical guides. A common first use case, cloning a repository from somewhere else, isn't even covered until Chapter 12, because understanding what cloning really means requires so much background.
Like most readers, I'm an everyday user of Git but not at all an expert. Even this everyday use is enough to make me appreciate the scale of the task faced by the authors. On more than one occasion, frustrated by some idiosyncrasy, I've cursed that Git is a terrific engine surrounded by a cloud of bad decisions. The authors might not put it quite so strongly, but they clearly recognize Git's inconsistencies (the footnote on p. 47 is one vicarious acknowledgement) and they gamely enter the ring anyway. As with wrestling a bear, the question is not "Did they win?" but "How long did they last?"
For the most part, they more than hold their own. You can sometimes sense their struggle over how to present the information, and one of the book's weaknesses is a tendency to fall too quickly into implementation-driven presentation after a basic concept has been introduced. The explanation of cloning on p. 197 is one example: the jump from the basics to Git-specific terminology and repository details is abrupt, and forces the reader to either mentally cache terms and references in hope of later resolution, or to go back and look up a technical detail that was introduced many pages ago and is suddenly relevant again. On the other hand, it is one of the virtues of the book that these checks can almost always be cashed: the authors accumulate unusual amounts of presentational debt as they go (in some cases unnecessarily), but if you're willing to maintain the ledger in your head, it all gets repaid in the end. Your questions will generally be answered, just not in the order nor at the time you had them. This isn't a book you can read for relaxation; give it your whole mind and you shall receive enlightenment in due proportion.
The book begins with a few relatively light chapters on the history of Git and on basic installation and local usage, all of which are good, but in a sense its real start is Chapters 4-6, which cover basic concepts, the Git "index" (staging area), and commits. These chapters, especially Chapter 4, are essentially a design overview of Git, and they go deep enough that you could probably re-implement much of Git based just on them. It requires a leap of faith to believe that all this material will be needed throughout the rest of the book, but it will, and you shouldn't move on until you feel secure with everything there.
From that point on, the book is at its best, giving in-depth explanations of well-bounded areas of Git's functionality. The chapter on git diff tells you everything you need to know, starting with an excellent overview and then presenting the details in a well-thought-out order, including an especially good annotated running example starting on p. 112. Similarly, the branching and merging chapters ensure that you will come out understanding how branches are central to Git and how to handle them, and the explanations build well on earlier material about Git's internal structure, how commit objects are stored, etc. (Somewhere around p. 227 my eyes finally glazed over in the material about manipulating tracking branches: I thought "if I ever need this, I know where to find it". Everyone will probably have that reaction at various points in the book, and the authors seem to have segregated some material with that in mind.) The chapter-level discussions on how to use Git with Subversion repositories, on the git stash command, on using GitHub, and especially on different strategies for assembling multi-source projects using Git, are all well done and don't shirk on examples nor on technical detail. Given the huge topic space the authors had to choose from, their prioritizations are intelligently made and obviously reflective of long experience using Git.
Another strength is the well-placed tips throughout the book. These are sometimes indented and marked with the (oddly ominous, or is that just me?) O'Reilly paw print tip graphic, and sometimes given inline. Somehow the tips always seem to land right where you're most likely to be thinking "I wish there were a way to do X"; again, this must be due to the author's experience using Git in the real world, and readers who use Git on a daily basis will appreciate it. The explanation of --assume-unchanged on p. 382 appeared almost telepathically just as I was about to ask how to do that, for example. Furthermore, everything they saved for the "Advanced Manipulations" and "Tips, Tricks, and Techniques" chapters is likely to be useful at some point. Even if you don't remember the details of every tip, you'll remember that it was there, and know to go looking for it later when you need it (so it might be good to get an electronic copy of the book).
If there's a serious complaint to be made, it's that with a bit more attention the mental burden on the reader could have been reduced in many places. To pick a random example, in the "Branches" chapter on p. 90, the term "topic branch" is defined for the first time, but it was already used in passing on p. 68 (with what seems to be an assumption that the reader already knew the term) and again on pp. 80-81 (this time compounding the confusion with an example branch named "topic"). There are many similar instances of avoidable presentational debt; usually they are only distractions rather than genuine impediments to understanding, but they make the book more work than it needs to be. There are also sometimes ambiguous or not-quite-precise-enough statements that will cause the alert reader — which is the only kind this book really serves — to pause and have to work out what the authors must have meant (a couple of examples: "Git does not track file or directory names" on p. 34, or the business about patch line counts at the top of p. 359). Again, these can usually be resolved quickly, or ignored, without damage to overall understanding, but things would go a little bit more smoothly had they been worded differently.
Starting around p. 244 is a philosophical section that I found less satisfying than the technical material. It makes sense to discuss the distinction between committing and publishing, the idea that there are multiple valid histories, and the idea that the "central" repository is purely a social construct. But at some point the discussion starts to veer into being a different book, one about patterns for using Git to manage multi-developer projects and about software development generally, before eventually veering back. Such material could be helpful, but then it might have been better to offer a shallower overview of more patterns, rather than a tentative dive into the "Maintainer/Developer" pattern, which is privileged here beyond its actual prominence in software development. (This is perhaps a consequence of the flagship Git project, the Linux kernel, happening to use that pattern — but Linux is unusual in many ways, not just that one.)
The discussion of forking and of the term "fork", first from p. 259 and reiterated from p. 392, is confusing in several ways. It first uses the term as though it has no historical baggage, then later takes that historical baggage for granted, then finally describes the baggage but misunderstands it by failing to distinguish clearly between a social fork (a group of developers trying to persuade users and other developers to abandon one version and join another), which is a major event, and a feature fork (that is, a branch that happens to be in another repository), which is a non-event and which is all that sites like GitHub mean by forking. The two concepts are very different; to conflate them just because the word "fork" is now used for both is thinking with words, and doesn't help the reader understand what's going on. I raise this example in particular because I was surprised that the authors who had written so eloquently about the significance of social conventions elsewhere would give such an unsatisfactory explanation of this one.
Somewhat surprisingly, the authors don't review or even mention the many sources of online help about Git, such as the #git IRC channel at Freenode, the user discussion groups, wikis, etc. While most users can probably find those things quickly with a web search, it would have been good to point out their existence and maybe make some recommendations. Also, the book only covers installation of Git on GNU/Linux and MS Windows systems, with no explicit instructions for Mac OS X, the *BSD family, etc (however, the authors acknowledge this and rightly point out that the differences among Unix variants are not likely to be a showstopper for anyone).
But this is all carping. The book's weaknesses are minor, its strengths major. Any book on so complicated a topic is bound to cause disagreements about presentation strategy and even about philosophical questions. The authors write well, they must have done cubic parsecs of command testing to make sure their examples were correct, they respect the reader enough to dive deeply into technical details when the details are called for, and they take care to describe the practical scenarios in which a given feature is most likely to be useful. Its occasional organizational issues notwithstanding, this book is exactly what is needed by the everyday Git user who wants to know more — and is willing to put in the effort required to get there. I will be using my copy for a long time.
 One of my favorite instances of this happened with the term "fast-forward". It was introduced on p. 140, discussed a little but with no mention of a "safety check", then not used again until page 202, which says: "If present, the plus sign indicates that the normal fast-forward safety check will not be performed during the transfer." If your memory is as bad as mine, you might at that point have felt like you were suddenly reading the owner's manual for an early digital wristwatch circa 1976.
 Though not absolutely always: one of the few completely dangling references in the book is to "smudge/clean filters" on p. 294. At first I thought it must be a general computer science term that I didn't know, but it appears to be Git-specific terminology. Happy Googling.
 (This is relegated to a floating footnote because it's probably not relevant to most readers.) The book discusses other version control systems a bit, for historical perspective, and is not as factually careful about them as it is about Git. I've been a developer on both CVS and Subversion, so the various incorrect assertions, especially about Subversion, jumped out at me (pp. 2-3, p. 120, pp. 319-320). Again, this shouldn't matter for the intended audience. Don't come to this book to learn about Subversion; definitely come to it to learn about Git.
 As long as we're having floating footnotes, here's a footnote about a footnote: on p. 337, why not just say "Voltaire"?
 Finally, I categorically deny accusations that I gave a positive review solely because at least one of the authors is a fellow Emacs fanatic (p. 359, footnote). But it didn't hurt."