Review use of the library catalog: Books & ILL
Search subject specific databases to identify journal articles on a topic.
Search for sources using citation info.
How you might think about sources:
Biographical or autobiographical information about the author(s)
Literary analyses of works
Historical events
Maybe this is oversimplified, but it's often the case that your thesis will involve historical events that may have shaped an author's life, biographical accounts of the author's life, and critical responses to the author's work.