The single best encyclopaedia is without a doubt Baensch's Aquarium Atlas. There are a series of volumes, but the first one will keep the casual aquarist going for years. Every professional in the field uses this book, and you'll find it on the bookshelf of almost every serious hobbyist as well. It contains very few errors.* It is, in short, the freshwater fishkeeping bible. Nothing comes (even remotely) close in terms of range of species, reliability, and depth. As a fishkeeping writer, it's the book I turn to more often than all my other books combined (and I have something like a hundred).
After a while even semi-serious aquarists tend to buy books focused on one particular bit of the hobby; Malawi cichlids, South American catfish, plants, etc. It almost doesn't matter which books you buy in this regard, provided you get some idea of the best authors. In the case of African cichlids for example, anything written by the likes of Ad Konings, Paul Loiselle or Anton Lamboj could safely be regarded as worth reading.
Cheers, Neale
*Figure-8 puffers being freshwater fish, and black widow tetras being peaceful spring to mind.
lol
