Anna Marie Johnson et al. have published "Dissertation to Book? A Snapshot of Dissertations Published As Books in 2014 and 2105, Available in Open Access Institutional Repositories" in the Journal of Librarianship and Scholarly Communication.
Here's an excerpt:
Only a small percentage of books published as dissertations were found in ProQuest and then subsequently in IRs. The number of libraries holding book titles with corresponding dissertations in IRs dropped between 2014 and 2015.