Having a book that is set (or written) post-1970 AND is in the public domain is quite difficult, so I'm going with the more liberal 1900 limit. I nominate two books:
- "At the Mountains of Madness" by H.P. Lovecraft. From user hudson on Feedbooks: "One of the best longer Lovecraft novels, a story of a doomed antarctic exploration party that uncovers the eldritch nightmares buried and sleeping under the snow." I've been reading a lot of Lovecraft recently, but haven't read this one yet.
- "Planet of the Damned" by Harry Harrison. From Feedbooks: "Hugo nominated in 1962 ... Brion has just won the Twenties, a global competition to test achievements in 20 categories of human activities -- but before he can enjoy his victory he's forced to leave his homeworld to help salvage Dis, the most hellish planet in the galaxy."