Sorry to appear to be arguing here, but two of your books are still available from the bookshops, so I'm not sure in what way you're hoping to "preserve" them. They are
"Powder River Jack and Kitty Lee's Songs of the Range (Cowboy Wails of Cattle Trails)" (Chicago, Chart Music, 1937) - available from Alibris $21, Country Bookshop $15.
Glasmaker, W. J. "The Big Roundup of Cowboy Songs" (1934, Amsco Music Sales Co., 1934) - available from Alibris for $21, Goddwill Books $15.
The other two don't appear to be available in bookshops but they are still in copyright and other copies will probably exist in libraries (I haven't checked).
Have you asked the authors (or their heirs) for permission to copy these books?
MMario - what you're doing is not the same. I checked the book you are working on and it appears to be OK from the copyright point of view. Also there don't appear to be too many copies of it around (though I might be able to find one if I knew what the right title was!)