I put together an online iCloud photo album with some pictures comparing books with the same or equivalent e-book on iPad Air and iPad Pro.
While the iPad Air is a better size for typical novels and articles, the iPad Pro is (IMHO) better for textbooks† (like technical books and RPG books) and comic books.
Digest-sized RPG books are often better on the Air. Although, the Pro does allow viewing two of them side-by-side.
Unfortunately, to get the two books side-by-side I had to load them into different apps. (In this case, iBooks and PDF Expert.) Although I haven’t played with PDF Expert enough to know what it can do. My PDF app of choice, GoodReader, hasn’t been updated to support side-by-side or to fully support the iPad Pro yet.
Being able to view two arbitrary pages in the same PDF side-by-side could be useful too. While you could do this with the two apps workaround, it would be better if a single app added such a feature. GoodReader has the ability to open multiple pages from one PDF in separate tabs, so that seems a reasonable addition to better support the iPad Pro.
But being able to view a book at full iPad Air size on one side of the Pro while having another app—e.g. Pages, UX Writer, Evernote, etc.—at full iPad Air size on the other side seems promising.
†Ironic that the word “textbook” seems á propos to me here since what distinguishes these from typical novels and articles is that they have lots of images, tables, and diagrams in addition to text.