Mountain bike trails that I would recomend for folks who are new to the sport or who need more experience on single-track are:
Cascade Canal Trail It's above Nevada City and its about as flat of a trail as you get around here. A very nice trail too!
Litton Trail It runs through the Nevada Union High School and Sierra College in Grass Valley.
Pioneer Trail especially between the 5-Mile House and White Cloud.
Confluence Trails in Auburn.
Foresthill Divide Loop Trail
Snow Mountain Flume off Willow Valley.
Bullards Bar Trail, the easist part is between Dark Day and the Vista Point.
The Downie River Trail in Downieville.
Empire Mine trails in Grass Valley {no map yet}.
The Washington and Alfa & Omega Diggins trails and dirt roads.
I hope to publish more trail maps for beginers as I have time...
Enjoy the trail,