From the announcement:
I’m pleased to announce that the next deadline for the NEH/Mellon Humanities Open Book Program will be September 26, 2018.
The Humanities Open Book Program is designed to make outstanding out-of-print humanities books available to a wide audience by making them open access. NEH and Mellon are soliciting proposals from academic presses, libraries, scholarly societies, non-profit publishers, museums, and other institutions that publish books in the humanities to participate in the Humanities Open Book Program. Applicants will provide a list of previously published humanities books along with brief descriptions of the books and their intellectual significance. Awards will be given to digitize these books and make them available as Creative Commons-licensed “ebooks” that can be read by the public at no charge on computers, mobile devices, and ebook readers.
Read more here.