Fishes of the World by Joseph S. Nelson is a standard reference for fishsystematics. Now in its third 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 a self-contained introduction, then after a short section on Chordata and non-fish taxa, the work gets down to the business of listing all the fish in a top-down fashion. Each family gets at least a paragraph, and usually a body outline drawing; large families will have subfamilies and tribes described as well. Genera and species are mentioned if they are notable, while the complexities of the higher taxa are described succinctly, with many references to the literature for difficult points.
The third edition predates the wide use of DNA analysis in taxonomy, and much of its classification will likely be revised.
Fishes have two problems to solve: one is physical, the problem of moving in the water: the other is chemical, the problem of keeping their body fluids at the right balance of salinity (saltiness).
Fishes need to be buoyant: if their bodies were too heavy they would lie on the bottom of the lake or sea and would have to work hard to swim.
Fishes are cold blooded, which means that their bodies are at the same temperature as the surrounding water.
Fishes of the World - definition of Fishes of the World in Encyclopedia
Fishes of the World by Joseph S. Nelson is a standard reference for fishsystematics.
It begins with a general overview of ichthyology, although it is not a self-contained introduction, then after a short section on Chordata and non-fish taxa, the work gets down to the business of listing all the fish in a top-down fashion.