One of the most important sources for the San Saba treasure legends is the 1905 book The Rise and Fall of the Mission San Saba by John Hunter. J. Frank Dobie used it, a lot, when he wrote the chapter on the San Saba Treasure in his book Corobado’s Children.
It was a limited run print for a Texas Rangers convention held in Menard that year. About thirty years later John’s son Marvin published a reprint of it. It is a hard book to find to say the least.
Well, it showed up in digital form here. That website has a number of scans of works I know are still in copyright, but this book is well out of copyright so no harm in making use of it.
Regarding Hunter’s work, I think where he was right, he was really right. Where he was wrong, well, he was really wrong. He had the Los Almagres mine being next to Menard, which it is not–it is clearly in today’s Llano county. He was not the first to write about the Tres Manos Bowie story, but he was the first to really elaborate on it, and the Tres Manos story was nothing but folklore.
But, he was the only person who saw the original ruins of the presidio before it was taken apart and wrote about it. He also visited the remains of the mission before the location of it was lost for several decades. And if it was not for this book, it may never have been rediscovered. Hunter had moved to Mexico during the Civil War to avoid conscription by the Confederacy, and when he returned to Mexico he spoke Spanish and had heard a number of rumors of silver on the San Saba. Hunter is to be ignored at one’s peril, even if not everything he wrote was perfect.
If you are interested in the legend enough to make it this far, then read that book!