Depends what you're after. A while back I
put together a list of good books that I, Bob Fenner, and some of the other Wet Web Media crew thought particularly useful. But I can't think of any one book that is all things to all people.
For example, Peter Scott's "Complete Aquarium" is great for inspiration, covering a series of different biotope tanks in great depth, in the process explaining things like planting, water chemistry, which rocks to use, how to landscape, and so on. But it doesn't contain all that many species. On the other hand, the "Baensch Aquarium Atlas, Vol. 1", is perhaps the best all-around book for cataloguing freshwater fish species, but in other ways its a bit dated and not all that easy to read.
A good approach is to think about what fish interest you. There are particular writers associated with particular fields of the hobby. Almost anything on cichlids by Paul Loiselle will be worth reading, or Ad Konings on Lake Malawi, or David Sands on catfish, or Frank Schaefer on oddballs. If you want a book about fish rather than fishkeeper, I cannot recommend too strongly "The Diversity of Fishes" by Helfman, Collette and Facey. It's pricey, but it's possibly the single best textbook about fish ever written.
Cheers, Neale