I read the already quite old book Think and Grow Rich by Napoleon Hill on the recommendation of several people. Wonderfully relevant still.

The book is public domain and can be read and downloaded from multiple sites. For example https://archive.org/details/NapoleonHillThinkAndGrowRich_201706.

Being interested in public domain books, I thought I’d make a list of sites where you can find public domain books, but of course that already exists: https://www.vice.com/en_us/article/qvq99b/how-to-download-the-books-that-just-entered-the-public-domain, for example.