There’s a lot of reading material out there. For the basics, I still recommend that students purchase K&R. People say good things about C Programming: A Modern Approach; I’ve only skimmed it. For advanced C I’ve not read a better book than Expert C Programming, though like K&R it is fairly old. The Practice of Programming is a really great book though it’s not completely specific to C. I haven’t read all of it but from what I’ve seen Modern C is a very good resource, with AFAIK the best treatment of undefined behavior of any C book. The C FAQ contains lots of good material.