Mainly field guides deal with plants and animals, but also there are field guides for identifying footprints, rocks and minerals, clouds, the stars and planets, and many other things. Using a field guide to identify something usually entails finding the unknown thing's illustration in the guide, then looking at its name and reading a little about it. Most field guides are absolutely packed with hundreds of illustrations and one never tires of thumbing through them.
When I think of all the pleasure that my various field guides have afforded me right around my home, on weekend trips, and major vacations, I just have to think that buying such field guides must be about the best investment a nature-lover can make!
Field guides reveal much more about the items they cover than merely their names. Bird field guides, for instance, describe each species' song, and tell what kind of habitat (woods, grassland, beach, etc.) each species prefers. Special quirks of behavior are mentioned, such as constantly bobbing the tail up and down, which is what a Spotted Sandpiper does. This kind of information can be critical when identifying birds in the field.
Even more useful are range maps. Let's say that you're in Montana and a goldfinch comes to your feeder. You go to your field guide and see that in the US there are American Goldfinches, Lesser Goldfinches and Lawrence's Goldfinches. They are similar to one another and you didn't really see all the field marks you needed to see when your bird was at the feeder. You just know it was some kind of goldfinch. Happily, your field guide's range maps show that only one species of goldfinch makes it to Montana. Moreover, as the sample distribution map above shows, it's in Montana only during the summer. Distribution maps are wonderful!
There are also field guides to rocks, clouds -- all sorts of things!
Here's something to keep in mind: Some field guides include illustrations and information on all the species of plants or animal they are treating, while others just include some. This is important to know if you want to be sure about your identifications, which we do. With field guides illustrating only part of a plant or animal group, you just never know whether you've identified something correctly, for you may have found a similar, closely related species, not illustrated...
Most field guides to birds, mammals, reptiles and amphibians, trees, and ferns include all species in the geographic region they claim to cover. However, if you find a pocket-size field guide to the insects of North America -- or to the wildflowers, weeds, grasses, or mushrooms -- you can bet that all species won't be considered.
That's because there are simply too many species in these latter groups to fit into a field guide small enough to be usable in the field. Though there are "only" around 850 species of birds found in North America north of Mexico, there must be around 100,000 insect species in the same area! It's just not physically possible to illustrate and describe them all in the 400-odd pages a typical field guide must be limited to.
This isn't to say that "incomplete field guides" are useless. They can certainly teach us the most common species. Also, they can introduce us to the basics of the art of identifying the group being considered. For example, small insect field guides can help us learn the various insect orders, and the most common families. It's at the genus and species level that things start getting sticky with incomplete guides.
Always be on the lookout for field guides limiting themselves to small geographic areas. For instance, if you ever find a field guide with a title something like "A Field Guide to the beetles of Smith Park," and Smith Park isn't far from where you live, you've got a gold mine, beetly speaking! Using such a field guide in the Smith Park area, you wouldn't have to wade through the jillions of beetles found in marshes along the coast, in the pine forests up in the mountains, and out in farmers' fields.