I guess I should also have said which is the one I use.
I actually have two, only one of which I would recommend to anyone.
I have the National Audubon Society guide and I really don't like it for most things. Its organization is probably great for someone who has no idea what they're looking at, everything is grouped by its appearance. But that means that if I wanted info about a clearwing I'd have to look for it among the wasps. So it's just not very useful if you do know what you're looking at. It also one section of the book with images and all of the information must be looked up elsewhere, which isn't extremely handy at all.
On the other hand, it has a much treatment of immature insects and non-insects than the other guide I have now.
Also, I have a bunch of other Audubon guides and I love all of those, it's just the insets I don't like much.
The other guide is the Kaufman guide which is just awesome. My only real complaint there is that it would be better if it treated only the western US, but that's only because I live in the west and it would be awesome if there were more insects included that I am likely to encounter. But this is definitely the guide that I would recommend every time.