I bought an old copy of William T Innes' Exotic Aquarium Fishes for $1 online. It's quite outdated (1950s or so) but it has beautiful photos and illustrations, and the text is really lovely to read. Innes goes into detail about a lot of species (his section on cories is so cute), as well as topics on care, feeding, breeding, plants, building your own tank, etc. There are even pronounciation keys for all the scientific names. It's hard bound with a very pretty colour plate of harlequin rasboras in just inside. Definitely worth it!