The Complete Joy of Homebrewing (Papazian)
Often called the "homebrewer's bible," Charlie Papazian's Joy of Homebrewing is the perfect place to start for any homebrewer. Filled with lots of how-to's, chemistry, recipes, and style guideline information, this is a book that be just as useful after your 10th brew as your first.
Not only is this book chock full of great information, it's presented in a very easy to understand and accessible way. We like it so much we include it in our beer starter kits.