Well here's some info that might be of help. You are correct about books with trail raitings they seam to be difficult to find but there are many, many books and videos out there covering Baja. the SideKick maps are great I've used them a lot for off-roading in Big Bear.
Off Highway Adventure Series 1
Sidekick Off Road
San Felipe, Baja Mexico Map
133 miles south of the U.S. border. All vehicles, beach riding, primitive and full hookup camping, mild climate, warm water.
[More Sidekick Off Road Trail Maps]
7 day a week x 24 hour a day
Fast & Secure On-Line Ordering
4X4NOW Price $2.70 (including handling)
http://www.rainbowgardensbookshop.com/desert.html