The best way to choose a field guide for insects is to read reviews by others who use similar field guides. An organization called Insect Identification for the Casual Observer rates some of the top insect field guides in the United States, giving top ranking to the "National Wildlife Federation Field Guide to Insects and Spiders of North America."
The "National Wildlife Federation Field Guide" gets the highest marks because it is easy to use and has excellent photography, succinct but to the point information and relative portability. It has a waterproof cover, which is useful in the field. It also contains articles on how to collect and identify insects as well as construct an aquarium-like habitat for captured insects.
Next, the "National Audubon Society Field Guide to Insects and Spiders: North America" is lauded for its thoroughness. It has brief, concise information on an amazing number of insects and spiders. This guide also has top-quality photography in full color. The book, small but thick, can slip into a back pocket in the field. The main drawback that did not give this book first place was the layout, as the photographs and text are arranged in different places. The third rated book, the "Kaufman Field Guide to Insects of North America," is considered a backup to the primary guides. It is not as portable as the other guides, the information is less useful and the photographs are not well laid out.