Dissertations and Theses are original research works. They can be a 'sneak peek' into scholarly ideas and findings at the cusp of popular scholarly discourse. They tend to have extensive bibliographies and can provide insight into the early work of a specific person, and can be helpful for biographical and historical research.
Obtaining dissertations can be a challenge. Sometimes they are immediately available as full text online, but when they aren't, turn to UMSL's Interlibrary Loan. We can likely help you get the materials you need by borrowing it from a library that has it. You can also try contacting the author. Some authors post their dissertations on their own website or have journal articles or a book, that draws heavily from their graduate work.
Photo of a handbound dissertation by Jonas Löwgren CC licensed