For books I've read I use Zotero. Easy to tag (read, want to read, which library has them) as well as organize and you can add notes including annotated pdf's

For books I've read I use Zotero. Easy to tag (read, want to read, which library has them) as well as organize and you can add notes including annotated pdf's easily. I use org-mode to track things I want to learn (i.e. online course, project, etc), and I archive the notes when I completed them. If I need to lookup something it's pretty easy.