I recently learned about Feminae and wanted to make sure I shared it sooner rather than later. Started in 1996, Feminae offers citations for journal articles, book reviews, and essays in books pertaining to the study of gender/sexuality in the Middle Ages. The index is hosted by the University of Iowa Libraries and can be searched like a typical library catalog, with both basic and advanced search options. Currently there are over 20,000 records in the index, and they appear quite carefully indexed by subject.
One thing to keep in mind is that the index does not include books written by a single author — the site’s maintainers encourage visitors to use a library catalog in these cases.