Do you know of any newer books in the public domain?

I know a lot of older books are in the public domain because of copyright law, so basically everything published before 1927 or whatever. But are there any more recent authors or publishers who have relinquished their copyright early and released their work into the public domain? Or any notable works that fit other criteria for entering public domain sooner than the typical “life + 70 years” timeline? How can I find books like this?

TLDR: seeking public domain books that were published AFTER 1927.

(Note: I am American and referring to US copyright law/public domain.)

by merlinsbeard4332