There are many books available for all levels of C programming skill. These can be found on the Stack Overflow website at https://stackoverflow.com/questions/562303/the-definitive-c-book-guide-and-list.
A note about Stack Overflow: I have found that most of the answers to questions available on the Stack Overflow website are often useful starting points; very rarely have I ever gotten a complete or totally correct answer from that site. It is useful, to be sure. Take what you find there and run the programs in your environment. Modify the solutions to your situation. But do not stop there. Most often, you will learn about something you had never considered—when that happens, keep questioning and experimenting for yourself.
On the other hand, I have also learned things from Stack Overflow that I'd never even considered before. So, it can be a rich and useful resource if approached carefully.
Another website worth exploring is comp.lang.c, Frequently...