Book level permissions
Book level permissions need to override platform settings.
There is no easy way to have the default be to preserve the last copy for online use, but have individual titles override that setting. Essentially, book-specific permissions only override loan/download days (and, the point at which an alert is set for low loan days remaining).
So, the only way to have most 1U books reserved for online access but specific books available for download is to set the download days to 0 in access permissions and to whatever you'd like the loan period to be using book specific permissions for the books that allow downloading. However, that would disable download for those access models entirely even when multiple copies are owned. There isn't currently a way to do something like this: disable downloads for ONLY the last available copy for all but this one book.
NB exchange with Paul Rehbein, Technical Support Consultant II at email.technicalsupport@proquest.com RE: 07997580

Thank you for your suggestion. The Ebook Central team is reviewing this idea to determine how it might fit into our future plans. We aren't able to provide a timeline for these ideas, but be sure to check back often and vote for the ideas you support in order to receive status and comment updates.
Best,
Sarah Breiding
Product Owner
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Jaimie Miller commented
TLDR: It’s a bit frustrating that book level permissions do not override default permissions.
The entire point of book level permissions is to override default settings.
Consider building in book level permissions that override default settings, so that we may adjust access according to our users needs.