It really doesn't have anything to do with old books as such. It's more about reading the actual thing rather than later commentaries and summaries on the thing

It really doesn't have anything to do with old books as such. It's more about reading the actual thing rather than later commentaries and summaries on the thing, if you want to know what the thing said. In the case of, say, CS, it would mean reading On Computable Numbers, With An Application To The Entscheidungsproblem rather than someone else's book/article about what Turing said in his seminal paper.