I'm genuinely curious to hear any specific changes you might make. I'm a librarian and I can say that Libby was a huge improvement in usability over the Overdrive app and we spend much less time on basic user support now. There are weird quirks for me as well, but I'm always hard pressed to say exactly what I would change.
It is a huge improvement over the low bar set by Overdrive. I use Libby everyday with my children and I think my three complaints are:
1. Filtering and categorization can be confusing: I have a difficult time finding books that are age appropriate for my daughter and available. I end up memorizing favorite authors as that is easier than the filtering.
2. The four buttons at the bottom are… interesting? The “library card” button the second from the left does… what exactly? I have never really understood the purpose of that button or why it is shaped like a card.
3. Figuring out how to exit a children’s book that is read “within” the libby app is inconsistent. I can never figure out how to reliably call up the UX to exit or change books.
Re. 2 I just checked: for the currently selected library, it shows you what looks like curated content from the library, to enable you to pick up on things that you might not necessarily do when just interacting with the app with searches and etc.