ZK Client-side Reference is the reference for client-side programming, including component development.
Client-side programming is optional for application developers. However, if you'd like to have more control of the client, please refer to the General Control and Customization sections. The other sections are more for component development.
If you would like to develop a component, you should read ZK Component Development Essentials first for the introduction. Then, you could reference this book if there is an issue.