Billy Jim Layton, Italian Music for the Ordinary of the Mass 1300-1450
I had the pleasure of meeting Billy Jim Layton only once before he died in 2004--he was kind and generous with his time. We mostly talked about composing and contemporary music (this was at a conference in honor of Reinhold Brinkmann, so it seemed appropriate).
When we talked about the dissertation he was suprised and tickled, saying, "I had no clue anyone ever read the thing," unaware that it was one of (if not THE) most cited dissertations in medieval music-- owing not only to its quality but also in the difficulty of finding a copy (it's on thin paper that doesn't photocopy and it's not in UMI) and that none of its results were ever published.
Layton generously allowed me to place a copy of his dissertation on the web here.