I found this list of books available through their Open Access program. There are about 1800 books currently, but unfortunately I wasn't able to find anything but an alphabetic list of titles. The book list can be downloaded in one of a couple of formats but I didn't find a quick way to separate them by subject matter. The couple I looked at were broken into individual PDF files with download links.
https://about.muse.jhu.edu/muse/open-access-overview/