On TAPoR: http://tapor.ca/tools/163

Bookworm is a free tool from the Harvard Cultural Observatory designed to generate data from searches on the Open Library and Internet Archives’ collections of public domain books. It allows users to search keywords, including in multiple languages simultaneously, and narrow terms by country, gender, subject and header. At present, Bookworm is restricted to searching books published between 1820 and 1920 due to availabliity. The tool produces a graph showing instances of each keyword over time, enabling easy comparison of results.