I have a lot of ebooks that I download for university research, hobby learning and friends who ask for help sourcing books. I put everything in my calibre library, which is great for metadata management (tip: I have it set so new books that I've just imported get a tag of "new", which I remove when I have processed their metadata. This allows me to chip away at ensuring the metadata is correct and good, even if I don't do it at time of import).

Anyway, at one point I found myself at risk of becoming overwhelmed by books, because if I'm wanting to learn some category theory, for example, I'd have multiple books that seem to be relevant. Some of them were recommended by programmers, some of them assume a higher level of maths background knowledge, some of them are more fun to read — once upon a time I might've known which was which, but if there's a significant gap between me downloading stuff and using it (which is often the case, I'm quite opportunistic with book recommendations), I may forget. Making a note of why I downloaded a particular book is something I've been trying to do more, so I can identify the useful things at the right time — the calibre notes field can work for that, but I'm still figuring out how to manage this in a wider sense because I do a lot of reading and it's easy to forget why I'm reading a particular thing. I think I have a calibre plugin to show which things I've read also.

Another related thing is that I will take a cursory look over a book when I download it, and I may delete it and not put it into my calibre library. This feels significant because downloading a book doesn't make it one of my books, 'taking it home' and putting it away on my 'bookshelf' makes it mine. In short, I try to be mindful in my curation activities, recognising that doing it in big clumps with my whole collection doesn't really work and that pruning little and often helps more.