WorldCat is a catalog of most libraries in the U.S. & Canada and many from other countries. Find books, DVDs, web sites, musical scores, archival material and more.
There are two versions of WorldCat, the subscription version from First Search, which is below and which I recommend, and the free version. If you find something you need while searching in WorldCat, and we don't own it, we can usually get it from another library. See InterLibrary Loan (ILL) to learn more. One great feature of the First Search version is that it automatically fills out your ILL request form.
Google Books is extremely useful, because it searches the complete text of a book. It can help you find books that may not come up in STARS, the library catalog. When you find a book that looks good, then do a TITLE BEGINS WITH search in STARS to see if the Library has it.
As of August 2013, Google Books contained over 30 million scanned books. Many from before 1923 are available full-text.