Vim has an overwhelming number of features. Its built-in help system and documentation are comprehensive and easy to navigate once you know what you're looking for, but knowing where to start is sometimes very difficult. The best you can hope for in a book is a broad outline to point the way toward features that you didn't know much about. Hacking Vim 7.2 achieves this goal.
No topic is covered in nearly the depth you'll find in the official documentation (or even on the Vim Wiki), but every topic is covered in enough detail to let you know that a feature exists and to point you in the right direction to begin using it. Most helpfully, throughout the book are references to things to look up in Vim's help system, as well as links to various relevant scripts on http://www.vim.org/scripts/.
This is not a book for an absolute Vim beginner; some familiarity with Vim is assumed. And for a Vim fanatic, much of the material may be common knowledge for you already. But any seasoned Vimmer will tell you that there are always things to learn about this editor, and I think nearly everyone will learn something from this book. For someone who uses Vim and is looking to master it, this book is a great starting point, though you'll still need to dive into the official reference material to really cement your knowledge.
The book starts on an odd note. Chapter 1 is a history of vi and the various vi clones released over the past couple decades. This information is interesting trivia and serves to give credit to programmers who paved the road to Vim, but it doesn't really help anyone "hack Vim" in any way. The book probably could've done without this chapter.
Chapter 2 deals with customizing the overall look and feel of Vim. How and where to edit vimrc is covered, with brief attention given to cross-platform issues. It covers the basics (changing font faces and colors, customizing menus and toolbars), as well as pointing out some more obscure settings, like highlighting the cursor row and column (creating a kind of "cursor crosshair"), and using the match feature to highlight multiple search terms at once. This chapter is a good foundation for later chapters and a good introduction for anyone who has never edited their own vimrc.
Chapter 3 is about text navigation. Sadly, the book doesn't go into as much detail on movement commands as I would've liked. The ability to move around and manipulate text quickly in Normal Mode by combining counts and motions/operators is one of Vim's most unique and powerful features, but it only gets a few paragraphs here.
There are some interesting key mappings provided, for example how to move up and down between "virtual" lines when lines are soft-wrapped. Search is covered briefly, both plain text search and multi-file search via vimgrep, but there's little information about Vim's powerful regular expressions, which I thought was a shame. Marks are discussed, both normal "hidden" marks as well as visible "signs", the latter being a feature I've never used.
Chapter 4 is about "production boosters" and covers a wide variety of topics. Much of the chapter is devoted to "templates" and "snippets", which allow you to build skeletons of commonly-used source code (with fill-in-the-blanks markers) that can be re-used when editing new files. A system for using these templates is built from scratch using Vim script, providing a clever and useful example of scripting in action.
Auto-completion is covered in a lot of detail. Some custom key mappings are provided to help make "omni-completion" in Vim a bit easier to invoke. This chapter also very thoroughly covers Vim's multiple copy/paste registers and how they work. Recording and using macros, pointed out as one of Vim's more overlooked features, gets a good, lengthy example.
"Undo branching" in Vim is wonderful, but difficult to understand. Chapter 4 gives a simple, step-by-step example of why it's useful and how it works. This chapter also briefly discusses folding, vimdiff, netrw (editing files remotely via SSH and other protocols), and ctags. There's lots of good stuff in this chapter and you're almost certain to learn something useful.
Chapter 5 covers text formatting, both using built-in Vim commands and by piping text through external tools like par and tidy. A lot of space is devoted to using Vim to prettify plaintext, for example by centering titles on a line, adding ASCII-art dashes for headers and making bulleted lists. If you edit plaintext in Vim often, this is probably a great chapter, but I didn't find much use for most of it.
For programmers, the book discusses the different indentation styles available in Vim and very briefly shows how to write your own indentation functions, and how to indent and reformat blocks or whole files of code all at once. "Paste mode" also gets a passing mention. Personally I think a programmer reading this book would've benefited from much more detail about Vim's myriad indentation and text-wrapping options and how they work together, as this can be one of the most frustrating parts of Vim to configure correctly.
I had high hopes for Chapter 6 and 7, which deal with Vim scripting, but I was largely disappointed. Chapter 6 deals with scripting basics, and is essentially a beginner's language tutorial. It explains which variable types exist in Vim script, how if/then/else works, how for- and while-loops work, how function parameters operate, and so on, but anyone who knows a modern scripting language will learn these things quickly without much effort. There's also some basic information about how to write a syntax-highlighting script from scratch, but there's not really enough information to allow you write one for a real programming language.
Chapter 7 is supposed to be about "extended scripting" topics, but serves largely as a style guide. It details how to structure a script to check for compiled-in features and Vim version number. This chapter touches briefly on using SID and PLUG to namespace functions, but the explanation and example left me puzzled. How to use the debugger and how to make Vimballs are both explored, and the book points out that you can use Perl, Python and Ruby to script Vim without going into much detail or giving solid examples.
If you're looking for any advancing information on writing your own functions in Vim script, you're mostly out of luck here. Previous chapters in the book do include some useful and practical functions, but those functions are never really taken apart or explained in detail.
Finally there are two appendices, one of which lists a bunch of games you can play in Vim (again this could've been left out of the book and I wouldn't have missed it), as well as examples of using Vim as a mail, chat, and Twitter client. There's also a feature-by-feature comparison of Vim to MS Visual Studio, showing that many of Visual Studio's abilities can be provided in Vim given the proper scripts. I thought it was an interesting demonstration that Vim really can do everything, just in case the reader had any doubts at this point. The last appendix is a style guide for keeping your vimrc clean, mostly via common sense and splitting your configuration into multiple files.
Overall, stylistically the book is a bit dry and humorless, but it's easy enough to read and it gets its information across clearly. There were a few typos and editing errors, including a few rather glaring typos in some code examples, but overall the author seems extremely knowledgeable about Vim. The best parts of the book are where the author says "this was useful to me personally, so here's how I do X". This book is clearly written by someone who uses Vim all the time, and most of the information provided is practical and immediately usable.
I do feel the book should've gone into more detail in many areas. At 244 pages, the book is short and gives a rather shallow view of many of Vim's features. But the book hits all the right notes and leaves few features entirely unexplored.
I'd recommend this book to any person who uses Vim and wants to explore features they may have been missing. There's nothing in this book you won't find in Vim's built-in documentation, but this book lays everything out in an easy-to-read format, and should serve as a good starting point to customizing and mastering Vim."