That, of course, will be a book digitized under the Partner Program, with the permission of the publisher. The contract for that has some problematic elements, in my view, but that is, I suppose, a ‘caveat emptor’ matter. Certainly no one is being opted in by default. And the contract is short enough, and clear enough, to read and digest in a reasonable time. The UK version is, anyway.
If Google had stuck to the Partner Program (and public domain books, of course) it would have done us all a service - itself included.
Authors whose publishers put their books in the Partner Program do need to take care, when the rights revert, that the publisher terminates or transfers the Partner Program contract for it. There is an author on the web complaining that although she has reverted her books, Google Books will still only talk to the publisher.