Designing Great Beers
by Ray Daniels. Statistical study of the winners of the National Homebrew Competition. Great fun for competition wonks.
This was the most helpful book when I was learning to brew. Although I am not a professional, I have adopted many of the practices outlined in this book and am now producing some delicious brews from grain and extract. The concepts are updated and clearly explained. I recommend this book be read/kept by all brewers as a reference and a must read for beginning homebrewers.