Well, the query didn't say what kind of short films they were researching
but one source is Leonard Maltin's "Selected Short Subjects: From Spanky to
the Three Stooges" (1983).  Silent comedy shorts are discussed in performer
bios and Gerald Masts's "The Comic Mind."  Books on music in film--such as
Krin Gabbard's on jazz--often cover soundies and musical shorts.
Avant-garde and experimental films are probably even better covered by
books like "New American Cinema," "Dada and Surrealist Film" or the Scott
MacDonald and Stan Brakhage books.
Lang Thompson
http://members.aol.com/wlt4/index.htm
 
----
Online resources for film/TV studies may be found at ScreenSite 
http://www.sa.ua.edu/screensite