The C Programming Language – Brian Kernighan and Dennis Ritchie
The first edition of this book is the first book on C that I’d ever read back in the 80’s. It is by far the best book on C out there. Short and sweet and full of simple examples. Co-written by the inventor of the language (and indeed co-designer of Unix itself), it is the Strunk and White of programming manuals.
The second edition covers ANSI C, which is a more modern version of the language.
Since most of Blender is written in plain C, I think trying to learn C++ will just confuse people. In many respects C++ is a very different language, although the two can be mixed by experienced programmers.
If you find a copy of The C Programming Language in a used bookshop, or possibly online (google is your friend), I recommend that you buy it and read it from cover to cover!
I still keep my beat up copy as a reference.