Oxford Music Online is an excellent general reference tool for music that via Grove Music Online contains:
MPAL also has all of these titles in print.
Dictionaries specific to American music will often contain entries on American people and topics not included (or not given much space) in the general reference works listed above.
These comprehensive guides to the literature on American music are a good place to start, and can lead you to bibliographies and works on more sepecific topics.
To find reference works or guides to research (also called bio-bibliographies) for a specific composer, there are a number of searches you can try in the online library catalog.
1. Search for the composer's last name and the phrase "guide to research" in a catalog Keyword search.
2. Search for the composer's last name and the term "bio-bibliography" in a catalog Keyword search.
3. Search for the composer's last name and the term "bibliography" in a Subject search