The National Institute of Informatics (NII), Japan, has released an SRU API for Bibdata-EBook, a database of Japanese e-books:
https://ebooks.erds.nii.ac.jp/primo-explore/search?vid=NII_EBOOKS
Please note that authentication is required to access metadata for domestic e-books.
If you are interested in using the SRU API from institutions in Europe, please contact Mr Koji Sakaguchi at sakaguti@nii.ac.jp