I really like
Manual of Fish Health By Andrews et. al. It covers a lot of info on exactly how water parameters affect fish and their health, as well as some good disease ID images and treatment recommendations. For example, it explains the biological processes that go on during nitrite poisoning (and the same with every other water parameter). It's a book I have seen highly recommended several times.
There are other books out there, some with microscope ID pictures if you're the kind of person who likes to do scrapes and check them under a microscope.
Surprisingly, I found the reviews on Amazon to be really helpful on this, although I didn't think they would be. I guess there are more fish freaks out there than I thought.