If you’re after a book to help with C, then two I would suggest are Zed Shaw’s “Learn C The Hard Way” and Kernighan & Ritchie’s “The C Programming Language”.
“Learn C The Hard Way” is a teach by example, type out this code and here’s what it’s doing type book.
“The C Programming Language” is written by two people who should know the language better than anyone, since Dennis Ritchie actually created the language. It might be an old book, but it’s still widely regarded as the de-facto C reference.
You might be able to pick up some of the basics of C by doing some C++ material, but they are not the same language and there are differences that you would need to know about and accommodate for between the two.
If your course is expecting you to learn C, I would suggest that you learn C.