While the focus of the open access movement has been on peer-reviewed journal research, scholarly output provided in other formats is also being made OA: for example, theses and dissertations, chapters in books, and even books themselves.
I often reference theses and dissertations on my principal blog, Forced Migration Current Awareness; you can browse these via the subject label "theses."
I also link to OA-available book chapters as I come across them. These are not designated with any specific subject labels; however, I have compiled a listing of OA book chapters posted as of January 2018. For post-2018 references, search for "chapter" in the blog, and then browse accordingly.
Finally, I maintain an "Open Access Books" page on my Forced Migration Library blog. To locate additional OA titles, check out the Directory of Open Access Books (DOAB) and OAPEN: Online Library and Publication Platform. The Open Access Directory also maintains a list of publishers.