My Kindle library has grown to almost 1,500 books over the last 10+ years with many still unread. I'd love to be able to get recommendations for just these books either based on tag or owned fields.

Right now we can't guarantee that we'll get good matches given any user's preferences combined with their owned books. That's why we only have the option to remove them for cases where they do in fact overwhelm the suggestions.

Perhaps we'll have an enhancement to our Up Next Suggestions feature — either within it or separately — where we'll be able to show you which books you reckon you'll enjoy the most at any given in time from your owned books pile.

Yes please! I was coming here to request this same thing.