Historical Newspapers has various references
Google Books Advanced Search turns up various references
Biographies of magicians (biography of Houdini has chapter "Bess: the magician's assistant, the magician's wife"
Magicians handbooks
Magic periodicals (in UMD Discover search subject "Magic tricks--periodicals"). The sphinx : a monthly magazine for magicians and illusionists professional and amateur can be found in Victorian Popular Culture, which has a section titled "Spiritualism, Sensation, and Magic."
Images: ArtStor has some posters showing magicians and assistants, including gruesome poster of Prof. Lowanda, also found here; illustrations in Magic 1400s1950s.
American Museum of Magic and other magic museums.
