Debug + Building a C++ Program from an existing project to add on lines as instructed

Hello, I would like to build a book program that will find data and show on the other end. We are already half way on the project and needs help to understand and complete. There is simple data constructors and modifiers that will be built. I would like to understand the code first before we implement for understanding the reason for each one. We can do this two ways, either step by step or you can finish the whole thing and explain at the end. Thanks