I think similar like Simplicio, but I would like to point you to Hanson's "C Interfaces and Implementations" which will show you how to organize your C code wel

I think similar like Simplicio, but I would like to point you to Hanson's "C Interfaces and Implementations" which will show you how to organize your C code well and how to implement robustly ADTs. It is pretty educative and enjoyable to read.