If a dissertation or thesis is published, you can cite it as a book. However, often, they are not published anywhere other than through dissertation and theses databases. In this case your citation in APA should look like these examples:

Edwards, B. (2008). Motivation and middle school readers: Graphic novels, comic books, and free
voluntary reading time (Unpublished doctoral dissertation). The University of Oklahoma. Retrieved July 26, 2012, http://search.proquest.com/docview/304486184?accountid=9783

Hughes, M. M. (2008). Politics at the intersection: A cross-national analysis of minority women's
legislative representation (Unpublished doctoral dissertation). The Ohio State University.
Retrieved July 25, 2012, from Permalink: http://rave.ohiolink.edu/etdc/view?acc_num=osu1217434642

Please visit our Library Citation/Styles Guide to learn how to cite different types of materials in different citation styles, such as APA, Chicago, Turabian, ACS, CSE, AMA, NLM and some others.