I use Kavita and KavitaEmail to organize and have a frontend for my books, and the latter to email them to my kindle if it's not on there yet. My kavita container is stopped most of the time because I already know what I'm going to read next and just need it up to sync or send new books.
Used to just have my library I exported from Amazon and ebooks com on a single folder on my NAS, kavita helped clean it up a bit.
I also tried audiobookshelf but mostly for audiobooks and podcasts and didnt quite fit my workflow I already had and liked using kavita and Antennapod.