I have very little information on William Stahl. Here is what I have...William Stahl was a prominent music publisher and teacher of guitar, mandolin, and banjo in Milwaukee from the late 1800s to the early 1940s. Most of the American guitarists of the 19th century taught and played mandolin and banjo in addition to guitar.
In my music library are a few of his books. Nonpareil Folio books 1 and 2, Newton's Conservatory Methods(published by Stahl), William Stahl's New Guitar Method. Books also by Stahl that I don't have include Star Collection of Easy Pieces for Guitar, Brunover's Easy Recreation Album for Guitar, J.C. Folwell's Famous Compositions for the Guitar. I would love to acquire more of Stahl's books as they are very well written for the guitar.