The subject of protected marine species is a large one, concerned with all aspects of biology and wildlife management. I will attempt to put various resources into context and a suggested order for visiting, to make the most of getting acquainted with the field.
This page will start small, and grow with time. As I get time to add to it, you'll be able to see it change, hopefully for the better.
The first stop on the guided tour is a text that describes how to get involved in a career in marine mammal science. Even if you aren't interested in a career, though, this text has a lot of information concerning marine mammals and organizations that deal with marine mammals. Strategies for Pursuing a Career in Marine Mammal Science is a brochure from the Society for Marine Mammalogy, posted here with permission.
Our next stop on the way to learning more about marine mammals is to see them as humans have been able to for centuries: when they strand on the beach. Visit with the Texas Marine Mammal Stranding Network to find out more about what happens when marine mammals strand, and what can be learned from stranded animals. Continue the same kind of lesson at the Marine Mammal Center, California's stranding network authority.
First-hand knowledge of cetaceans at a detailed level is not yet a common thing. A book that tells about personal experience of very uncommon depth is Sam Ridgway's Dolphin Doctor. The site has pictures of dolphins at work and play, and an anecdote about how "Tuf Guy" got his name.
Just about everybody would like to swim with dolphins, so here is a first-hand account. Pictures of some of the dolphins mentioned can be seen here.
There are many questions that arise when studying dolphins. Many of them are answered in the Dolphin FAQ.