@zwol, I'm a just student who learnt C by myself around five-ish years ago, and the one I picked up was Beej's Guide [0]. It seems that it's still incomplete as of today, but it helped me picked up the core concepts and made me comfortable with it (from casual competitive programming to occational Python extensions). It's short, it's lighthearted, and it's actively updated: it's so different now that I should probably read it again d-;