Excerpt from book preface:

This book describes an alternate class methodology that provides complete data hiding and fault-tolerant run-time type checking of objects in C programs. With it, you will produce code that contains fewer bugs.

The class methodology helps to prevent bugs by making it easier to write C code. It does this by eliminating data structures (class declarations) from include files, which makes a project easier to understand (because there is not as much global information), which makes it easier to write C code, which helps to eliminate bugs. This class methodology, which uses private class declarations, is different from C++, which uses public class declarations.