Some links. I'm aware of 3 published trail book series by Roger Mitchell, Charles Wells, and a series called 4WD Adventures.
Roger Mitchell uses a Class I-V system. Charles Wells groups trails into easy, medium, difficult and lists the trails in an increasingly difficult gradient within those groups. The 4wd Adventure series uses a 10 scale which I believe is the same as the "Colorado 10 scale" referred to in one of the links.