Frommer's is probably the first guide book I grab when I am traveling international. I have to admit it never dawned on me to use a Frommer's guide book for travel within the U.S. When we did our northwest car trip a couple of years ago I depended upon AAA tour books, information I had gotten from each state plus tips I printed off of the Internet.
For this trip I had picked up used Frommer's Frommer's Arizona and the Grand Canyon and Frommer's New Mexico guide books at a local book sale. And I can't believe what a help they were. Using them in conjunction with the AAA tour books made it easier to select hotels, restaurants, places we wanted to see and things we wanted to do.
Whether you buy them new or buy them used (at most 2 to 3 years old), I highly recommend adding a Frommer's guide book to your collection if you will be touring any part of the U.S.