Anyone who has some level in computer sciences has probably stumbled upon at least one book from O'Reilly. They are well known for their books on computer security, flavors of *NIX and whatever else can be associated with computers. Their Hack series, where this book is released under, has seen a wealth of titles, and more are close to release. Just some of the titles available are "BSD Hacks", "Flash Hacks", "Linux Server Hacks" and "Network Security Hacks".
All the books are made out of an large collection of 'hacks'. Each hack can be thought of as an on itself standing bit of information that enables you to accomplish a certain task. Each hack is listed as one of three different complexity levels: beginner, moderate and expert. IRC Hacks comes with over 400 pages of such hacks.

Let me explain you with an example how the book works in practice. Instead of a chapter about a bot, where you will finally end up with a single fully working bot, there are a series of chapters dedicated on bots (there are chapters on logging bots, search bots, fun bots, etc.). Each chapter contains a wealth of hacks which each ending up with a fully functional bot, doing what the hack was written for. There are for examples hacks that let you make (and understand!) a bot that logs joins and parts, logs URL's people mention, Google searching bots, etc. etc. Of course in all hacks the source code is explained in detail for you to understand what is happening, and where appropriate tips are given on taking things further ("hacking the hack"). Especially handy since the book is not following one programming language, but several including Perl, Python, shell scripting, Java, etcetera, giving you the experience of "trying out" some languages so you can possibly choose one to dive in and master it.

The hacks cover a wide range of topics from encryption, installing and configuring an ircd (Unreal and Beware ircd's), IRC'ing over telnet, setting up SSH tunnels, etc. etc. Take a look at the O'Reilly's website for the book for a full table of contents. Doing so, you might also stumble upon my only dislike about this book. There are hacks that explain how to "Highlight Lines in mIRC", or "Add Timestamps to mIRC Messages". It could be misinterpretation from me, but to me it seems very unlikely that people who have problems with configuring mIRC, considering with all the help files it comes with, will ever buy such a book that will learn them to code bots that interact with the medium IRC. Luckily, the amount of space used for more interesting hacks seriously outweighs this, so not much is lost.

Time to go back to the first paragraph of this review and the question whether should you spend money on this book or not. To answer this you should ask yourself a few other questions first.
Do you consider yourself pretty confident in a programming language, and does RFC 1459 and other IRC related protocols not hold many secrets for you anymore? Probably not then. When you need to look up the occasional problem you'll dive into Google or the RFC in question and you'll be able to look up whatever you need to solve your problem.
Are you a user who is just happy to have a chat with his friends and you don't really care about the underlying technology of IRC? You are not interested in making your own custom bot? Then this book is probably not your thing either. Get online, have fun chatting, look no further (but you will most likely not be reading this website then either!).
For anyone between these two extremes and with a keen interest of taking his IRC experience a step further, this book can be of great help. Yes of course you can find all the information written in this book online. But to get the same amount of information together as in this book, it would take a tremendous amount of time. Besides, getting information on a scripting language is one thing, but then utilizing that information and write a working IRC bot is a whole different thing. You have all the information ready to be soaked up by you, with all the irrelevant information cut out. And you just can't beat having a real paper and book to flip through either. And while doing that, to occasionally bump into something you simply did not realized before. And that can be your starting point developing and enhancing your experience you get from the medium of IRC. Happy exploring!