I'm currently re-learning C, and I've been going through one of the books that they recommend (C Programming - A Modern Approach).

Whilst "A Modern Approach" is a great book, it's hard for me to keep focused as I'm not really implementing anything other than the exercises and projects (for which there are too many IMO, and I struggle to get them all done).

I've been working through Build Your Own Lisp in parallel, and it's great fun. I get to work on an actual project, and any holes I find in my knowledge can usually be filled in via A Modern Approach.