A dissertation is required of doctoral study; it is a lengthy, formal document that presents new research, and that argues in defense of a particular thesis.
Dissertations can be useful for in-depth research.
Image source: The Wellcome Library, London. Public Domain.
Request a dissertation via Interlibrary Loan, or Tipasa. Tipasa is a service that will provide you with materials that LVC does not own.
WorldCat Dissertations will provide a Request from LVC Tipasa link on each of the results you find.
You can also request a dissertation from the Interlibrary Loan link on the library's homepage.
With Tipasa, you'll either get a PDF of the dissertation sent to you electronically, or a hard copy that will need to be returned.