They used much higher-quality paper 300 years ago, so those books don't fall apart. Somewhere in the 20th century they used really crappy acidic paper for the mass-market paperbacks. If you're looking at sci-fi novels in hardcover form, this doesn't describe those; they generally used high-quality paper. I have a bunch of those from the 70s-80s that are fine too. Go look at the crappy romance novels from the 80s, and you'll see a very different story.

Cheap pulp-based paper was a creation of the 19th century.

The 1898 Report of the Librarian of Congress has a long appendix title "The Durabilty of Paper" addressing just this concern:

<https://babel.hathitrust.org/cgi/pt?id=mdp.39015036735036&vi...>