This field guide has become our undisputed favorite. It's relatively new (2007 publishing year!) and comes complete with a waterproof cover(!). Not only does this guide cover the basic and exotic North American bugs but it also covers millipedes, centipedes and ticks in equal detail complete with imagery. Not only that but the guide also has great articles on how to use the guide, building your own aquarium-type ecosystem for any bugs you catch and breaks down the families of bugs with a history on each. Collecting and identification tactics are all covered in this neat little find. A MUST BUY for any insect identification freak. Our ONLY caveat on this guide is the fact that the binding is not very friendly to constant usage, meaning that over time, your guide will really start to show some wear and tear along the spine. If anything, buy two so you can use one in the field and keep one in pristine condition at home.
UPDATE 8/8/2007: Because this field guide is not ring bound, be prepared to wear the binding down if you're using it quite a bit and opening the book to the extremes. Maybe best to keep a field copy to wear down and a home library copy.