I am aware of three fictional stories about the San Saba legends, that is, stories that concede that they are only stories.
The first San Saba-ish story from the Texas Hill Country is popularly reckoned to be Burred Treasure, a short story O. Henry wrote in 1908. He may have pondered the local treasure stories while he was living in Austin, Texas in the 1880’s. It did not mention the San Saba treasure by name, but had elements common with the San Saba stories: a man writing down the directions to ten mule load of Spanish treasure in the 1860’s, for a twentieth century treasure hunter to pursue.
The earliest story that refers to the San Saba mine by name is The Red Paper, by C.C. Hotchkiss, a 1912 novel. No meaningful information on the legend (it actually put it some distance away from Menard and had the Texans covering it up), but it shows how wide-spread the legend was in pre-WWI America.
More recently is The Bowie Secret, by Bob Balch, a 2013 novel that goes from Jim Bowie up to the present. A work of fiction, but included some real people and places from around Menard. Some of the same people I met with a couple of years later. Mr. Balch is a Texas attorney and his take on how messy things could be, legally, if someone ever found a stash of silver is enlightening.
Of course, there are many books out there that are de facto works of fiction, and some of them have influenced the folk lore and legend concerning the story. Most of the books about Jim Bowie are, frankly, works of fiction where an ounce of history, if even that, was spun into a pound of book. Jim Bowie’s Lost Mine by M.E. Francis (1966) is a prime example.
Will finish this post with a cartoon from 1945. J. R. Williams was a popular cartoonist in his day. He drew cartoons about his experiences, from being in the cavalry to working in a machine shop. The cartoon below shows how wide-spread the San Saba legend was in those days.