As is so often the case, I think a great many writers used J. Frank Dobie's Coronado's Children (1930) as their primary - if not only - source for the story of the loot from Monterrey. See Chapter X "Los Muertos No Hablan."
Prof. Dobie ("Notes") recommends 3 books about the outlaws involved - including the outstanding Helldorado by William M. Breakenridge (who attended three hangings in his lifetime - none of them with the benefit of judicial order), Walter Noble Burns' Tombstone (great reading; includes a chapter on the treasure; should, however, be considered a novel until proven otherwise); and Lorenzo Walters' Tombstone's Yesterday.
Some researchers have not been able to locate any Mexican government records of a robbery of the Monterrey mint.
The wonderfully-named Zwing Hunt, among other hardcases mentioned in the story, certainly lived. There are 21 references to him in Ramon F. Adams' cornerstone bibliography Six Guns and Saddle Leather (Norman, Oklahoma: 1969). Before you spend too much time with any outlaw or gunfigher book or pamphlet, see what Mr. Adams has to say about it.
Finally there is the fascinating Historical Atlas of the Outlaw West by Richard Patterson (Boulder, Colorado: 1985). Many of the questions asked on the TreasureNet forum could be answered - or at least the answer could be started - by referring to this unique volume. Mr. Patterson's account of The Smuggler's Trail (page 9) relies on "Smugglers' Trail" by Robert L. Thomas (Frontier Times, August-September 1967).
Hope this helps!
Good luck to all,
~The Old Bookaroo