diseases of tropical fish by Herbert R. Axelrod (expensive classic book)
The interpet manual of fish health (there are a few different editions with varying authors)
The A-Z of tropical fish: diseases and health problems by peter burgess and mary bailey (good value but pictures rather than illustrations in the main)
The best book I have found to date is The Manual of Fish Health authored by Dr. Chris Andrews, Adrian Exell, Dr. Neville Carrington and Dr. Peter Burgess. Firefly Books, 2010.