The serious fisherman, like the good coach or general, does not just rely on the quality of his equipment and skill -- he or she wants to scout the opponent. That's in essence what a field guide is for: To let you know what types of fish are native to a given zone. These guides also help you prepare by telling you where to go to find the species you're after and by teaching you what you'll need to catch them.
The most popular guides cover continents, the North America books in the section below being the biggest examples. Other authors will concentrate on a fish population in a more limited region, the Eastern seaboard, for example. Be careful not to be confused by fish encyclopedias or tropical fish guides. These books, which specialize in finding and identifying interesting species, are spectator guides and will not help you fish. Remember that what you're looking for is, in effect, a hit list.
Seek out a book that has a good index, quality illustrations and covers your region of interest. Also make sure it's small enough size so you can carry it along on a trip. If you can't find a smaller book to your liking, pick a larger (which do tend to be good) and do your studying beforehand and either take notes along with you.