I'm echoing my other comment a little bit but I've been following Shepherd and thought I would comment since you're here. I like a lot of your ideas but I haven't quite managed to get recommendations that consistently sound interesting. I think it has something to do with authors and it would be interesting to know how you're thinking about how authors or writing styles are related.
For example, I like Bill Bryon and have read most everything he has written. He's good at making subjects I am not interested in captivating. For example medical history and anatomy in his book The Body.
I would love to read other books that make the uninteresting interesting but when I put The Body into Shepherd (or most recommendation engines) I mainly get other books about anatomy. This makes sense on the surface, but I've already read a book on anatomy. I'm ready for something else interesting.
I realize that might not be very actionable or useful, but when I am looking for something new to read I don't know what I am looking for. I just know the "vibes" books and authors I like, and often find that recommenders default to similar subjects only. Maybe this is difficult or impossible to capture, but it's been nagging me as something missing.