Perform your search, then select "Books" under "Resource Type" on the left side of the results screen.
Dissertations are the product of the academic research done to obtain a Ph.D. degree. A thesis is the equivalent product to obtain a Master's degree. Check with your instructor to see if these are acceptable sources for your research. For a full list of databases with dissertations and theses, including their descriptions, visit the A-Z List of Databases.