LOTS of single track trails (so I've been told). I'm exploring the jeep realm now and am enjoying stuff. A handy web site for 4x4 trails, but certainly could be used for single track work, is
www.TrailDamage.com (as in damage to your vehicle from the trail).
www.FunTreks.com also makes 4x4 trail books but does have some/minimal reference to motorcycle/offroad trail use.
You can stop in to the NFS offices for forest service maps as well, some paid some free.
If anyone wishes to ride along with me on a trail ride, let me know and I'll holler when a ride is planned.