depends on what your interest is, I think most of Napoleon hill's stuff is free on there for general self improvement. Go to folkscanomy, they have everything by category, from there you can some crazy stuff, British ww2 self defense manuals, guides to make a small town completely self-sufficient, the usual anarchist cookbook type pdf for making guns and ammos with scrap and pipes, involved guides on music, math, coding, fascinating stuff for cultivating rare skills.

For this sub, though, there are a lot of classics on there free, legal(?), and literally two clicks away, less involved and sketchy than the typical free .pdf online hunt, we should find something interesting on there and do a weekly or monthly bookclub on this sub. No better way to self improve than having routinized intellectual effort. E.G. a nice scan of Nietzesche's "Will to Power" is on there, love or hate the guy he has ideas everyone should tangle with, it would be fun to find 5 to 10 other users on here and start a discussion, the book is saturated with topics that have never been more relevant. But we could start with something lighter as well, some light sci-fi that's easy to read, then discuss the message or big idea at play