Add "no collection" collection
Collections are great. I use them to separate my novels from my non-fiction, and to split my non-fiction into Programming, Data Science and UI/UX, etc.
However, I have no way of finding out which books aren't in a collection. This is particularly important when Humble release another book bundle and I can't avoid buying it! When I add the books to my collection then I can't tell which ones aren't in the existing collections. Bugs gnome-documents#15 (moved) and gnome-documents#16 (moved) make it hard to re-tag everything, as I have to do it one-by-one.
It doesn't have to be a fake collection on the "Collections" page, but it would be useful to have a way to search "All books that aren't in a collection".