Dissertations are book-length scholarly works submitted for completion of the Ph.D. They aren't "published" in the same sense as books and articles, but can be useful tools for identifying new scholarship and bibliographies. Theses in the U.S. are typically submitted for completion of a Masters degree, although terminology differs by country.
UC's open access repository. Contains books, journals, working papers, conference publications, postprints, theses, and dissertations.