Mobiles size:
I do not think people use the editor mode in mobiles, but if so, is hard to read. Maybe we have to focus in tablets and if in mobiles have bad experience… well, we can not offer a solution for all the users at once.
Book info allow to change the url of the CPT Book-information. Do we whant that? I do not know if it matters, but if somebody do a plugin for PB, to change that would be an issue?
Likewise, in Aldine 2.0-RC.1, we hide all subjects which are not applied to the currently displayed books (which sometimes means the subject list is empty):