IDGAF about "user retention." 

A better experience is just a very simple, clean, easy to understand UX that just works. I know how that should function at the user. The bank end stuff is what I don't know Jack about. 

A clean, simple, straightforward user interface will do the most to "keep users coming back."

You only need to stick to:
-automagic book lookup by title, author, barcode, or ISBN 
-automagic cover lookup for said titles, as an option. I know some don't care for the clutter of book covers in a clean list view.
-three lists: reading, to read, read
-ratings upon completion
-it's not hard, I guess, to also allow written reviews but I don't think that's necessary for a first iteration
-nostr connectability to the whole ecosystem, but specifically to be able to share links to books, reviews of books, and curate a separate list of connected npubs to "follow" and possibly another list of npubs to "push notifications to" when I book is finished/reviewed/added to the list "to read" list
-further iterations might include a user curated and "recommended list" that others can easily add to their "to read" list, in part or total

But at this point I really should be doing my fiat mining job instead of building out all the app features. 

My point is that I really don't care about concerns of retention. That's not at all a metric that matters to me, as a user.