For a larger that average examination of SLC's platform technique,I'd
recommend "The Trouble Begins at Eight," written by Fred W. Lorch,
(Iowa State University Press, 1968). There's a whole chapter devoted to his
speaking style, as well as a decent amount of commentary by
contemporaries of SLC who attended his lectures.
More information is included in another chapter devoted to SLC's
reactions to different audiences. Together, they form as comprehensive a
picture of Clemens's speaking technique as I've seen so far.