Something that I've really struggled with is that I wish it were easier to make notes of where you've gotten a recommendation from. Like let's say I'm learning Python and there's a dizzying amount of books available for that. Someone I personally know and respect gives a glowing recommendation of a book, so I download it and process it's metadata in calibre or similar. Then a Reddit thread has a dump of free books. I download and import those. Then a blogger I really like recommends a few books, so I retrieve any of those I haven't got yet and import them. Time passes, and I go looking in my library for a book to learn Python with, and I get overwhelmed by choices and I can't remember which of them were good and for what reasons.

I have a similar problem with bookmarks, because sure you can add tags, but tags are best when you've got a limited vocabulary, to maximise reusability. What if I just want to write a brief note about why I deem this worthy of saving, so that I'm not confused when I'm skimming over bookmarks months or years later.