@FrustratedReader
On linux you can use find to search for books.
This example will look on home folder, recursivelly, for epub files modified at least 30 days ago, but skip the ones modified more than 360 days ago.
Code:
find ~/ -type f -iname "*.epub" -mtime +30 -mtime -360
You can change that to look for "*.mobi", "*.docx" or whatever. You can use -atime instead of -mtime to search for last accesed timestamp instead of last modification.
I hope it works for you.