https://doabooks.org/

DOAB is a community-driven discovery service that indexes and provides access to scholarly, peer-reviewed open access books (currently ~ 50 000) and helps users to find trusted open access book publishers. All DOAB services are free of charge and all data is freely available.

Where to start, if trying to implement this: https://www.doabooks.org/en/resources/metadata-harvesting-and-content-dissemination