• Topical encyclopedias such as The Dictionary of the Middle Ages (1982), The Encyclopedia of the Middle Ages (2000), The Encyclopedia of the Renaissance (1999), or Medieval Italy: An Encyclopedia (2003). Reference works such as these should NEVER show up in your final paper bibliography, but they're a good place to get your bearings and pick up some basic bibliography. Available online at Brepolis.net are the International Encyclopedia of the Middle Ages and the Lexikon des Mittelalters (useful for bibliography even if you don't read German).
• The Cambridge Histories Online are generally excellent, and include major reference works on every topic from American history and warfare to music and literature (not to mention the New Cambridge World History of Human Disease).
  • The New Cambridge Medieval History (7 vols., 1995–2004) covers all sorts of topics, and its bibliographies are both excellent and recent.
  • If your topic veers into Late Antiquity, consult vols. 13 & 14 of the Cambridge Ancient History as well as the early volumes of the Medieval History.
  • For the early modern period, you'll want the early volumes of the New Cambridge Modern History (there is no separate Cambridge Renaissance History), but be aware of publication dates: the most recent edition of volume 2 (The Reformation) was published in 1990, which isn't so bad, but the most recent edition of volume 1 (The Renaissance) was published in 1957, which means its usefulness will be quite limited.
• If you use their search criteria intelligently (such as by limiting your search to books in English published in the last ten years), large university catalogues such as HOLLIS (at Harvard) or MELVYL (at the University of California) can tell you what the most up-to-date research on any topic is. ILL or UBorrow a couple of those books and mine their bibliographies for your topic.
• Two other great resources at Brepolis.net are the International Medieval Bibliography and the Bibliographie de Civilisation Médiévale (not limited to works in French).
• Two final useful databases are 1) the ITER Bibliography, which includes materials on topics a bit further into the early modern period than the Brepolis resources (up to AD 1700) and 2) Kubikat.org, a catalogue for anything vaguely related to art history or material culture covering both books and articles (basically, it's the amalgamated catalogue for the four greatest German art history libraries in Munich, Paris, Florence and Rome).
• Worldcat is a great resource for figuring out how you can get a particular book, but a little too comprehensive to be really useful for general exploratory searches.

Three words of advice as you get started:

• 1) BE VERY AWARE of the dates of the works you consult. A book published in 1950 will give you neither an accurate sense of current scholarship on a given subject, nor an up-to-date bibliography. (This is not to say all books published before 2000 are useless—far from it—but part of your assignment is to formulate a sense of the bigger intellectual picture, something you won't get from checking a couple of decades-old books out of Cook Library.)
• 2) Speaking of which, make generous use of interlibrary loan and UBorrow (the statewide borrowing system, which offers you "X items you can request statewide" in the upper right corner of the basic NCF catalogue search-results screen). A great shortcut is Worldcat's "Request through ILL" button. If you have a choice, use UBorrow in preference to ILL since it's faster (a couple of days vs. a couple of weeks, often).
• 3) Advanced tip: If you're not sure about the reliability of a particular book, the best way to evaluate it is to use Worldcat or JSTOR to look up a few reviews of it by other scholars. "A few" is preferable to only one since you never know if the one you're looking at is by someone crazy, or a particular rival or enemy of the author in question.