My professor told me to search for 19th century books in something called NUC Pre-56.
The National Union Catalog, compiled by The Library of Congress, locates copies of books published prior to 1956 in American libraries. It is a large set of books shelved on the walls of the 3 west stacks Reading Room in Widener.
Although a search of OCLC's WorldCat database and the Library of Congress online catalog retrieves many records listed in the print editions of the National Union Catalog, a number of records are still unavailable. A 2008 article in College & Research Libraries, "The Proportion of NUC Pre-56 Titles Represented in the RLIN and OCLC Databases Compared: A Follow-up to the Beall/Kafadar Study" (PDF, 362 KB), estimated that 25% of NUC Pre-1956 records were not listed in WorldCat. (To read the 2005 Beall/Kafadar study, click here.) It is unlikely that WorldCat has reached 100% inclusivity in the intervening years, making a search of NUC an important supplement to WorldCat.
In addition, due to the manner in which the retrospective conversion of the LC's old card catalog to online form was undertaken, NUC continues to include some entries for works in that Library's collections not listed in their online catalog. Consequently, it should still be consulted in any comprehensive book search.