I used this book, by Michael Dawson, as a preparation for going into my project where I developed a simple 3D shooter in Unity. While Unity used C# instead, this was still a good introduction to C-style languages. The book itself covers basics such as variables and control loops, as well as functions, references and pointers. It also covered the details of classes and object-oriented programming, including inheritance, polymorphism and abstract classes.

A lot of these topics did come in useful later when programming my game, especially the object-oriented stuff, due to the object based nature of Unity programming.