Technically, GET_ACCOUNTS is required to use the AccountManager service, not to avoid knowing the user's primary email address. Applications are free to be stupid and leak information via globally-accessible storage.

I know it feels nice and vindictive to bash Google the Company for making stupid mistakes, but Google Books, like Gmail and Maps, is a wholly-separate team from the core Android team, thus it is a reasonable expectation that they can make the same mistakes as any third-party application developer.