I just find Kindle too closed. I root and modify my ereaders (hardware too).
Many people like Calibre. I can't be bothered.
I keep all my books in one directory on my desktop and just sync my ereader over ADB.
I've got a finite number of books ~500 and the rest are library books.
When I borrow a book it's at the top of my library app anyway.
See also:
https://www.mobileread.com/forums/fo...play.php?f=123