Although at first the idea might seem silly, a bibliography of bibliographies can be very useful. They act as general guides to a subject which can in turn point you to more specific guides relevant to your needs.
The following books are general music research bibliographies organized by topic which may offer starting points for research, lists of primary sources, or more detailed bibliographies on your topic.
Bibliographies of individual composers are classed in ML134 and are arranged alphabetically by composer. Most of these items are in the Reference section, but some are also upstairs in the general collection. The types of items found here include thematic catalogs (lists of composers' works with their opening themes), which usually include information about manuscripts or early editions, and bio-bibliographies, which combine biographical material with bibliographies of primary and secondary sources.
ML134 houses several particularly extensive series devoted to research guides and bibliographies, including: