How to brew by Palmer is the first book you should buy. You only need one how-to book and Palmer's is the best. It is a little dated (it probably needs a section on BIAB) but its still a great reference to have.
If you want a good recipe book, Brewing Classic Styles. Most recipe books suck and this one has its flaws too but it delivers a great overview of the BJCP guidelines and provides a "to style" recipe for each subcategory.
Brewing Better Beer by Gordon Strong. A lot of people hate this one but Strong's book gives you a lot of great ideas for improving your beer. It says its for "advanced brewers" but if you have a basic understanding on how to brew and your beer doesn't completely suck, you are "advanced" enough for this book (if you truly are an "advanced brewer" you will probably hate this book). It doesn't give you step by step instructions on anything but tells you what you need to do at a high level and leaves it to you to figure out how to accomplish it on your system. It also covers topics that most books don't touch like blending, competition brewing and bottling, hangover prevention, post fermentation adjustments, cold steeping etc...and unlike every other homebrewing book, it explains brewing water chemistry in a simple, practical way (don't even waste your time looking at the water section in How to Brew, it will only confuse you)
Seriously !
