Part 2 of C for Yourself is called “Using C” and should be read after you are familiar with basic C concepts. It covers practical topics that make it possible for you to write real programs. The features discussed in these chapters are provided in the QuickC run-time library, which, as you may recall from Part 1, is not part of the C language itself.
While Part 1 was designed to be read sequentially, Part 2 is topical. So you don't need to read its chapters in any particular order. If you are new to C, however, it is recommended that you begin with Chapter 11, “Input and Output,” which describes how to read and write data, and process files. Similarly, if you're not familiar with QuickC graphics, you should read Chapters 13–15 in order.