K&R still the best book on C. The language hasn't changed all that much over the years.

The C Programming Language (Second Edition) by Kernighan & Ritchie. Available online for free:

https://ia903407.us.archive.org/35/items/the-ansi-c-programm...

You can find other C books and online resources to get up to speed on "modern" C features (C99), but 95% of K&R still applies.