Start with CLIO Catalog to find books, journals, e-books, databases, and items in all formats in Columbia University Library collections. Titles held by the Law Library (and not already held by other Columbia Libraries) are now included in CLIO. (Use the Educat catalog to find books and journals in the Teachers College Library collection.)
WorldCat contains over 179 million records for items held by the vast number of libraries belonging to the Online Computer Library Center (OCLC). It includes records for books, manuscripts, websites and internet resources, etc. Searching WorldCat is an excellent way of seeing beyond the range of what Columbia has collected to the larger universe of what is actually out there.
One handy way to get an overview of scholarly literature on the subject of your research is to consult:
Oxford Bibliographies (Oxford University Press)
A series of articles on a wide range of academic subjects across all disciplines. Each article identifies, describes, and evaluates important books and journals for a specific subject. "Every article in our database is an authoritative guide to the current scholarship, written and reviewed by academic experts, with original commentary and annotations." (from publishers description)
ProQuest Dissertations & Theses Global
"The world's most comprehensive collection of full-text dissertations and theses. As the official digital dissertations archive for the Library of Congress and as the database of record for graduate research, PQDTGlobal includes millions of searchable citations to dissertations and theses from 1861 to the present day together with over a million full-text dissertations that are available for download in PDF format. . . . The database offers full text for most of the dissertations added since 1997 and strong retrospective full-text coverage for older graduate works."
EThOS : e-theses online service (British Library)
"EThOS is the UK’s national thesis service which aims to maximise the visibility and availability of the UK’s doctoral research theses. . . . EThOS aims to provide:
• A national aggregated record of all doctoral theses awarded by UK Higher Education institutions
• Free access to the full text of as many theses as possible for use by all researchers to further their own research."
EThOS includes records for theses from all UK PhD-awarding institutions, but does not yet hold all records for all institutions.