For anyone who want to learn more about fish I highly recommend reading: Fishes an introduction to ichthyology. By Peter B. Moyle and Joseph J. Czech, jr.
Books on ichthyology can by very heavy reading for a layman (like myself), but this book is toned down a bit when it comes to the technical language used. So it is much better for a "beginner" in that regard.
It roughly consists of these parts: structure and form, systematics,genetics,evolution, a presentation of different types of fish,zoogeography, ecology and more.
This book made me avare that the blood of fish is in direct contact with water through the gills. Think about it. Whatever is in the water is also in the blood of fish.
I particularly find the section on evolution interesting.