I love field guides. One should own a lot of field guides, not just to things you might go out in the field to see and identified, but just to browse through.
David Ebert, Sarah Fowler and Marc Dando have produced A Pocket Guide to Sharks of the World It is put out by Princeton, which does excellent guides (this is part of their Pocket Guide series).
From the Princeton site, about the authors:
David A. Ebert is the program director for the Pacific Shark Research Center and a research faculty member at Moss Landing Marine Laboratories. Sarah Fowler cofounded the UK Shark Trust and the European Elasmobranch Association. She has been a member of the IUCN Shark Specialist Group since 1992 and is currently vice-chair of international treaties. Marc Dando is a freelance artist specializing in marine ecology and wildlife.
Don't go to sea without this book, just in case you come across a shark.
This is a standard Petrides/Peterson style guide, with plates showing various sharks, and associated ID text. The front matter includes guides to fins and teeth, some basic information on biology and reproduction, the usual overview of external anatomy so you know what the terms are (so you know what a caudal keel is, and such). There is an extensive guide to teeth. There is a species checklist, an excellent glossary, and a list of species. Here's a PDF of the book's Introduction.
There are 501 species of shark known today, and all of them are in the book.
One of the coolest things about the book is how they show relative size. Sharks vary a great deal in size across the diverse species. To show the size of the sharks on a given page, the authors use an image of a human next to the shark.
There really aren't any other field guides out there like this, as far as I know. In my experience, if you come across a shark in most waters, it is highly likely to be one of a small number of common species and you don't really need a guide to determine what it is. You just need the old salt down on the pier to tell you. But beyond the first couple of common species, the next shark or two down the line in commonness may be any of a large number of sharks, and for that you need a guide.
Sadly, but out of necessity, the guide includes information on identifying sharks from the fragments that are cut off and sold by shark hunters and poachers.
Sharks are cool, and this is a cool book.