If you're trying to have your server return different translations or localized versions of the same books (in other words, the same resource from a RESTful perspective), then use Accept-Language because the resource is the same but the representation is different based on the client's needs.
However if you're trying to return completely different books based on the client's locale (say, returning books written in French if you know that the user is in France) then the URIs should be different since different resources would be returned. At this point, you're talking more about a query request more than anything else. For what it's worth, the /books/en
approach sounds reasonable. Another approach would be to add the locale or language as a resource parameter to GET as /books?lang=en
.