The Number 1 , must have, for any area, in my opinion, is the Delorme Atlas and Gazeteer for your area. Living on the State line, I have two, one for Arizona and another for Southern California.
As for other books, I've always found the local library a good place to start. If you have interests other than pure 4-wheeling, as I do, don't overlook books on minerals, old mines or Ghost Towns. Many will have maps and good area descriptions. I've also found quite a few trail maps just cruising around on the web. Most relate to old mines. Even if your're not interested in old mines or rockhounding, the guides will usually get you into some very scenic country.
I don't know how things are in your neck of the woods but, out here, BLM maps cover many of the trails and side routes, as well. Unfortunately, they don't rate the difficulty of the trail as some of the books do.
I was just looking over Wells' website,
www.funtreks.com , and noticed that you can order the books through him. While his prices are a bit higher than Amazon's, for an extra $3 you can get the books spiral bound. He also shows updated trail information on the site.