If you have your Harvard ID card, you can make use of BorrowDirect Plus! This program allows you to access other libraries that partner with Harvard in the BorrowDirect program. The BorrowDirect On-Site Borrowing page provides more information about access and borrowing privileges, and the steps to take to activate your account. Remember to ask about ID requirements before you visit the library.
If you don't live near Harvard or BorrowDirect institutions, visit your local public library, especially since they can acquire materials for you through their Interlibrary Loan service.
WorldCat: A collective catalog of records of any type of material (books, periodicals, scores, films, recordings, etc.) cataloged by over 41,000 member libraries, primarily but not exclusively, from libraries in the United States, but extending to 82 other nations. There are more than 47 million records.