BASE Bielefeld Academic Search Engine https://www.base-search.net/
COAR Confederation of Open Access Repositories https://www.coar-repositories.org/
CORE Open Access research papers https://core.ac.uk/
Digital Commons Network http://network.bepress.com/
DOAB - Directory of Open Access Books https://www.doabooks.org/en
DOAR - Directory of Open Access Repositories https://v2.sherpa.ac.uk/opendoar/
DOAJ - Directory of Open Access Journals https://doaj.org/
Open Access Digital Theological Library https://oadtl.org/
Google Scholar https://scholar.google.com/
Mason OER Metafinder https://oer.deepwebaccess.com/oer/desktop/en/search.html
MENDELEY - https://www.mendeley.com/?interaction_required=true
National Digital Public Library of America https://dp.la/
Paperity - aggregates papers & articles https://paperity.org/ see https://paperity.org/about/
Post Reformation Digital Library (15th to 18th centuries) http://www.prdl.org/
ResearchGate - https://www.researchgate.net/
AOSIS https://www.worldcat.org/search?q=AOSIS&qt=results_page
Archive.org https://www.worldcat.org/search?q=Archive.org&qt=results_page
Hathitrust https://www.worldcat.org/search?q=Hathitrust&qt=results_page
Internet Archive https://www.worldcat.org/search?q=Internet+Archive&qt=results_page
Intechopen https://www.worldcat.org/search?q=Intechopen&qt=results_page
DOAJ https://www.worldcat.org/search?q=DOAJ&qt=results_page
DOAB https://www.worldcat.org/search?q=DOAB&&dblist=638&fq=
AOSIS: https://aosis.co.za/book-collection/
ARCHIVE: https://archive.org/
Australian Islamic Library https://www.australianislamiclibrary.org/
Bloomsbury Open Access https://www.bloomsburycollections.com/search?newSearchRecord&browse&product=bloomsburyOpenAccess
BCcampus OpenEd https://open.bccampus.ca/browse-our-collection/find-open-textbooks/
CCEL (Christian Classics): https://www.ccel.org/
CORE: world's largest collection of open access research papers 209,692,405 https://core.ac.uk/
deGruyter open access https://www.degruyter.com/search/advanced
DOAB - Directory of Open Access Books https://www.doabooks.org/en
DOAR - Directory of Open Access Repositories https://v2.sherpa.ac.uk/opendoar/
FreeBooks4Doctors - http://www.freebooks4doctors.com/
Google Books http://books.google.com/
Hathitrust Digital Library https://www.hathitrust.org/
Intechopen https://www.intechopen.com/books
Internet Archive - https://archive.org/
IA Open Library - https://openlibrary.org/
JAIN eLibrary https://jainelibrary.org/
JSTOR - Open Access books https://about.jstor.org/librarians/books/open-access-books-jstor/
Lyryx - https://lyryx.com/subjects-2/
MDBI Books - https://www.mdpi.com/books
NTSLibrary.com - http://www.ntslibrary.com/theology-PDF-books.htm
Open Access Digital Theological Library https://oadtl.org/
Open Access European Networks - https://www.oapen.org/home
Open Edition Books https://books.openedition.org/catalogue (many French resources)
Open Textbook Library https://journalarchives.jisc.ac.uk/home
OUP Open Access titles https://global.oup.com/academic/open-access/titles/?cc=us&lang=en&
Project Gutenberg http://www.gutenberg.org/ebooks/
Princeton Theological Commons https://commons.ptsem.edu/
Sacred Texts https://www.sacred-texts.com/index.htm
ScholarWorks https://scholarworks.gvsu.edu/
ScienceDirect https://www.sciencedirect.com/browse/journals-and-books?accessType=openAccess
Springer Link: https://link.springer.com/search?facet-content-type=%22Book%22&package=openaccess
Taylor Francis Open Access https://www.taylorfrancis.com/search?openAccess=true
Theology Gateway https://theologygateway.info/
UPenn Digital Library of Online Books www.digital.library.upenn.edu/books/
Wikibooks https://en.wikibooks.org/wiki/Main_Page