NeverTooManyBooks.
I have several books cases and shelves where I store books, this has helped me a lot to have a catalogue of them.
I just scan the book code, the app gets book info, and I assign it to a place where it is physically stored, like, bookcase 1 in office room, shelf 3 on main room, etc.
I have it installed from Droid-ify which is like an app to have more sources for f-Droid.
https://apt.izzysoft.de/fdroid/index/apk/com.hardbacknutter.nevertoomanybooks
Also I want to recommend Birday, it is a very simple birthday manager, and you can import birthdays from your calendars.
I intend to use this to remember birthdays and actually call people on their phones.
https://play.google.com/store/apps/details?id=com.minar.birday