My wife doesn't understand what it is that I enjoy about 4-wheeling. For me, it's the combination of getting out in the mountains, enjoying the views, and when possible getting an understanding of the history of the trail that we're on. When I got my Jeep, the first book I got was "Jeep Trails to Colorado Ghost Towns" and I've been hooked on the history part ever since.
Recently, I've been studying the history of Boulder County looking for stage coach lines and trying to understand the history behind the trails that I'm most familiar with. Today at lunch I went to the Carnegie branch of the Boulder Public Library which specializes in local history. There, I found a copy of "The Switzerland Trail of America" which describes the railroad whose bed we now use as a 4x4 trail known as the Switzerland Trail. Tons of great pictures.
Searching their online database for historical maps, I ran across a USGS Historical Trails series and just found them online. These maps include stage coach lines, old railroads, old roads, town site names, and much much more. These maps are big (10-40 MB PDFs).
Here's the top-level page,
http://pubs.usgs.gov/sim/2004/2828/
The "Denver" Map which includes roughly the area from I-70 to Boulder,
http://pubs.usgs.gov/imap/i-2639/Plate1.pdf
and the northern quad from that map,
http://pubs.usgs.gov/imap/i-2639/Plate2.pdf
Here's the Greeley Map that includes some of Boulder and Larimer Counties (Nederland, Estes, etc),
https://store.usgs.gov/yimages/PDF/I...X2_Quad_CO.pdf
There's a lot of supporting documents that go with these too. This is the gold mine of information that I've been searching for.