If you look at the maps and books section
of this site, you'll see a guide book called The Playground Trail
by Lee and Jane Whitely. It does not have a lot of modern maps to find the old road, but does have several of the old auto trail maps of the highway. Part of the NPtoP was the Yellowstone Highway (not Trail), and there is a book by Lee Whitely on that as well. It's described on the same maps and books page. One of the things I'd like to do is make a road guide, and to make a road trip (or several) along the NPtoPH.
There is one section fairly well mapped and described, and that's the National Old Trails Road section from Los Angeles, CA to Holbrook, AZ. Most of that became Route 66. In the guidebooks, you want to look for the National Old Trails Road or 1926 Route 66 alignments. For the long Arizona section, I would recommend Route 66 Across Arizona : A Comprehensive Two-Way Guide for Touring Route 66
, but it seems that Amazon is no longer carrying it. For the whole section of 66 along the Highway, your best bet is Jerry McClanahan's EZ66 Guide for Travellers
. You can find links and info on Route 66 books and maps on my Recommended Route 66 Resources
I will have a page on the Highway on this site, but as always, I'm doing all this a little at a time.