Gutenberg is nearly all books that have lapsed into the US public domain by dint of being published 95+ years in the past. Which broadly explains why you hit nothing for 3d printing.

As another commenter said PG is almost all books from 95+ years in the past due to copyright law in the US. We partner with a sister organization, the World Library Foundation, who have a self-publishing portal for modern works by authors who wish to put their own work in the public domain. You might want to look there for more modern material. https://self.gutenberg.org