I have, however, figured out that it is much more productive to pull up a pile of books and go "okay, I have to get rid of at least 10% - 15% of these, which 1-2 are going?" than to agonize over how much joy each one brings me. I can pull out my 1-2 least favorite much more easily than deciding how much I *really* need every one. So we're going with that method instead.

That is frankly why the Kondo method does not work for me with books. I can sit there for half an hour and rationalize and agonize about how no I NEED something, and realize I am being ridiculous, and then feel bereft if I put it in the Nope pile. It works pretty well for everything else! Not books.