AAA has smaller regional directories that are free as long as you are a member. They are compact and have most of the same listings as Trailer Life has without the bulk. They also list state park and national forest campgrounds, not just AAA campgrounds.
If you are interested in National Forest Campgrounds, there is a guide that some folks put together that is a little out-of-date as far as the open/close dates go (1st printing 2001, updated 2003) as they were for a particular season. However, they do provide detailed info re: the access and terrain and sites. See:
http://gorp.away.com/dow/default
The info is all available online, but we bought a book for the Rocky Mountain Region to take with us. It helped us to find acceptable campgrounds in the area. However, you need to know the National Forest name first to find anything. We used the book to find a nice campground in the Big Horn Mountains with electric hookups and we found a campground near Laramie, WY for an overnight stay that was true to their description, right down to the bad road leading into that campground.
Here is the US Natl Forest website.
http://www.fs.fed.us/ You can find what campgrounds are available in a particular area and there is a link for reservations if the campground does take reservations. From there, you can select on state or national forest for further info.
I never can believe the ratings in the large books, but they do help to find campgrounds in a particular area to further research. I ususally also go to microsoft's terraserver
http://terraserver.microsoft.com/
to check out the terrain. Granted the aerial photos are a bit fuzzy and out of date, but they give a good idea of whether the sites are close togehter, shaded, etc. They also have topo maps. You have to go to "Advanced Find" and then type in a place name (the closest city, state for example) and then select which map you want to see. From there you have to navigate by direction until you find your desired location. They have pretty good resolution for some urban areas, but we typically don't want to camp near any urban area.
We have printed out some of the maps for use on our trips, especially the topo maps.