How to Brew always 1st reading. I read the online version twice before doing my first batch, and when I bought the book I read it cover to cover in a couple of nights. It's the Bible. I recommend a second copy to keep under your pillow.
New Lager Brewing is like an advanced version of How to Brew. He gets into topics like step mashing, yeast culturing, etc. Lots of practical information in there.
Radical Brewing by Mosher is a great book to read to learn about beer styles and get your ideas going for styles you may want to brew you haven't thought about brewing. He also has a lot of great recipes in there. My Wit recipe and my Tripel (in my dropdown) both are derived from recipes he presented in that book.
Designing Great Beers I haven't read cover to cover as I see it more as a reference book. I read the chapter on the style I'm going to brew before I finalize my recipe and brew it.
Those are my top 4 books.