Just as many other tropical fish hobbyists, I have quite a few books-primarily reference books-that I use to identify and learn some essential information about the biology, habitats, and behavior of the fish that I choose to keep.
The Axelrod Encyclopedias are excellent, The Amano books on planted freshwater aquariums are also very good. When it comes to the one reference that I refer to most frequently-it is the Baensch series. They are throrough, contain much more info than any equivalent book, often in the articles that open each volume, and have the best combination of photos, habitat info, and scientific taxonomy on the individual fish specimens than are to be found in any other reference for the general public.
Each new volume expands their usefulness, and although pricey-especially in hardback-they are worth every penny. They are a quality product and not only are indispensible in a practical sense of fish keeping, but add a range of information and credible knowledge that increases anyone's appreciation of this fascinating hobby.
If you are a tropical fish hobbyist, beginner or expert, buy these books!