Additions were made to several editions.
The original 1910 edition by John A. Lomax was reprinted in 1911 and 1915; Abebooks lists three at $104-225, the latter for a first in good condition.
Title: "Cowboy Songs."

Additions were made to a new edition, 1916-1919.I have a 1925 reprint which was reasonable in price.
A thread here at Mudcat discusses the additions and their dates of inclusion; it is much cheaper to buy one of these and use the Mudcat information to isolate those added after 1910.

The 1938 edition was a major revision, many additions; it was reprinted through 1966 (or later?). The title page became "Cowboy Songs and Other Frontier Ballads; Revised and Enlarged," but the cover title remained "Cowboy Songs." Alan Lomax was added as co-author.