The links below provide resources to e-Books in different disciplines that are available to read and download free of charge.
1. Springer Link eBooks:
https://link.springer.com/search?package=openaccess&facet-content-type=%22Book%22&utm_source=springer&utm_medium=referral&utm_content=RMarketing&utm_campaign=BBKK_4_CE02_SpringerOABhometoSL
2. Open Access eBooks:
https://openaccessebooks.com/ebooks.html
3. Search the Directory of Open Access Books:
https://www.doabooks.org/doab