Web developers should focus on understanding the technical content, but also be familiar with the general content. That is, the entire book will be relevant to you.
If you are not a web developer, depending on your background and level of comfort with web technologies, you may choose to skip over the technical parts, presented within the content in blue boxes and marked as Technical. Review the Table of Contents for a full list of topics. You can focus your study on the general content, completing the activities, setting up a Web Accessibility Auditing Toolkit, and, having finished the book, come away with a good understanding of the requirements for developing accessible web content.