Fishes of the World by Joseph S. Nelson is a standard reference for fishsystematics. Now in its fourth edition, the work is a comprehensive overview of the 25,000-plus species known to science.
It begins with a general overview of ichthyology, although it is not self-contained. After a short section on Chordata and non-fish taxa, the work lists all known fish in a top-down fashion. Each family gets at least one paragraph, and usually a body outline drawing; large families have subfamilies and tribes described as well. Notable genera and species are mentioned, while the complexities of the higher taxa are described succinctly, with many references for difficult points.
The fourth edition was the first to incorporate the wide use of DNA analysis, revising many earlier classifications.