My friend thinks book suggestion lists based on perceived quality/popularity are “worthless.” He only likes reading to “grapple with ideas.” What book suggestion list can we put together for him to fit what he’s looking for?

This is the message he sent me after I shared an /r/suggestmeabook thread with him:

book lists are worthless to me, because i never pick up books because im craving a good story or to connect with what’s popular.

99% of my reading is for grappling with ideas. I’ll pick up a book when i think it’s required to help me deepen on some thinking I'm struggling with.

What books can we suggest for him that fit what he's looking for?