Almost every book I’ve read in Books is from libgen. Download, open-in, and it’s on all my devices. Reading progress syncs and everything.
The best electronic comic book experience I’ve seen is from a free app (optional one-off IAP that does nothing—I wish it had higher tiers, the author is doing god’s work and I’d have given them $50 or something, no question). I put related cbr and cbz files in a directory in icloud, they show up seconds later in this app, it quickly figures out what they are (no clue how it’s as accurate as it is) and organizes them, and I get to reading two pages at a time in landscape on a 12.9” iPad.
The only Apple content I pay for is Apple Music, because it’s so damn convenient and I listen to way more music when I have it. I used to put my own music files in the exact same app, though, and that worked fine.