@mhoeber The UX is still being designed for this story by @talbs .
Brian's figuring out how to display the textbooks on this page.  He may choose to list them in an embedded unordered list within "Textbooks".

Yes, you are right that textbook items within a textbook collection cannot be sorted at this time.  One can relocate an entire textbook collection, but not a single textbook item.
Perhaps in the future, we can allow users to reorder textbooks within a textbook collection.  But because of the underlying data-model, it would be difficult to move textbook items outside of its collection (and there may not be a user need for it).

Also, note: there is discussion of possibly modifying how the Textbook tabs appear in LMS.  Instead of each one occupying its own space, they may be listed as a sublist within a single Textbook tab.