Start with CLIO to find books and journals in Columbia University library collections. (Use the Pegasus catalog to find books and journals in the Law Library collection and Educat for resources in the Teachers College Library.)
If the book is not in CLIO, then try Borrow Direct.
If not in Borrow Direct, request the title through Interlibrary Loan (you can check to see what other libraries have the title in WorldCat, the largest online catalog in the world).
To find dissertations, start with Proquest Digital Dissertations. Also check Dissertations & theses. UK & Ireland and DART, Europe's E-theses portal.
Other essential resources include: