The good books, unlike the good podcasts, can rarely be reduced to a single forum comment. You don't read them to cite them as a zinger in an online back-and-forth. You read lots of them, and you cross-reference them with the world around you, to slowly build up a view of the world that's irreducibly complex. You read them to escape yourself and your times -- the exact opposite of "navel gazing", in a sense.

Most books add to "the shared pool [of] meaning", as you say. Pick any one; I didn't have a specific one in mind. The commenter to whom I was responding is in a state where pretty much any well-written book about history would help them out a lot. Something written before 1980 might be especially illuminating.

It might take many books, if they want their comprehension of history to actually be "hardcore".