Group permissions apply where both the user and the document belong to the same group. For managing ebooks, you may have one group for protected documents and another (separate) group for users. That way all users cannot access all ebooks, or any single ebook until assigned an "ebook" permission.
So for any document that you want to open to all users of a group, move the document and the user to the same special group.
Another way to provide individual ebooks to individual users with using group or ebook permissions is to add the user in the document's whitelist. However, such options can complicate your management overview and it is strongly recommend, that to avoid headaches later should things get moved around, that you stick to simple group and ebook permissions.