Worldcat Local provides seamless access to millions of books through the Boston Library Consortium and research libraries worldwide.
BLC delivery time: usually 2-3 days
Interlibrary loan delivery time: usually 7-10 days, sometimes less
ILLiad is the library service which coordinates borrowing from other libraries across the US and, occasionally, around the world. The login/password is your UTLN, the account most Tufts community members use to get their webmail. For articles delivery time is usually two days or less (and the article most often arrives as a PDF in your email); for books delivery takes a little longer, usually 7-10 days.
Worldcat has well over fifty million records, covering thousands of libraries worldwide. It isn't everything in the world, but it is a very good index to what exists in research libraries. If it's here, it's almost certainly available by Interlibrary Loan. (Dissertations and things of which there is only one copy in existence not included).
Worldcat basic version - good for quick lookups, not so good for browsing or looking for a specific edition of something
Worldcat power tool version - more complicated, but includes some things not included in the web version and very much easier to be precise with
Theses and dissertations are frequently a good place to start doing a serious literature review. Tufts has access to them in several forms, depending on your need. Note: if the dissertation you need is not available through the sources below it may be possible to order by Interlibrary Loan.