Here's a link to find out how to determine if a book (or work) is under public domain, though it got confusing to me after a while; it might be easier to google "is X public domain". https://www.biblio.com/blog/2010/07/how-to-determine-if-a-book-is-in-the-public-domain/
And some more information about what you can do with public domain works https://gatekeeperpress.com/reprinting-and-selling-public-domain-books/
Also, if you're just wanting to read some of those books, here's a link that tells you about entire digital catalogues of free public domain books you can access https://www.vice.com/en/article/kz4e3e/millions-of-books-are-secretly-in-the-public-domain-you-can-download-them-free