Everyone has different opinions on what is the best, but my most commonly used books are:
- How to Brew
- Designing Great Beers
- Brewing Classic Styles
- Braukaiser (not a book, but probs the best free brewing resource available online)
- Yeast - the practical guide to beer fermentation
It depends on what kind of beer you like to brew. My favourites are anything by Graham Wheeler, preferably with Roger Protz, but they would not be the best for brewing lagers or American styles. (Except for the 1995 Brewing European Beers, which is now out of print)
Cut my brewing teeth on Graham Wheeler's books. They are perhaps a little lacking in information on Yeast and water chemistry. When they were written there was a lot less information on, and much lower availability of yeasts, and what it takes to manage water properly. Braukaiser in another really good online resource. Anything by Charlie Bamforth and the late George Fix is well worth reading.
My most used book would be A Handbook of Basic Brewing calculations, the real nuts and bolts of brewing at any scale!