In addition to the International Dictionary of Films and
Filmmakers, v. 3, there are Zasu Pitts filmographies in the index
to The Motion Picture Guide (Cinebooks, Inc., 1987) and in the
companion volumes Twenty Years of Silents and Forty Years of
Screen Credits (Scarecrow Press, 1970 and 1971).  These
filmographies do not seem to be identical (though it's hard to
tell without doing an item-by-item comparison), so you may want
to check them all if you have access to them.
 
Cindy Murrell
Univ. of Calgary