Online reading is nice but I enjoy books too. I usually don't buy, I'm a library user mostly. I just recieved the new book "Loaches Natural History & Aquarium Care" written by several of the old timers at Loaches.com. I'm sure there are species specific books on many fish but some may be seriously out of date with obsolete info.
Diana Walstead book on planted aquaria sounds interesting, the no water change theory.
Google around, there's lots of scientic papers available if you're into really in depth studies, abstracts if you not. Genetics? Natural habitats? Ichthyology in many varieties.