The book "The definitive guide to HTML5 WebSocket" is a short book with a great content, if you are looking for web sockets this book is definitely a good start.
Chapter 1: Introduction to HTML5 WebSocket
Chapter 2: The WebSocket API
Chapter 3: The WebScoket protocol
Chapter 4: Building instant Messaging and Chat over WebSocket with XMPP
Chapter 5: Chapter 5: Using Messaging over WebSocket with STOMP
Chapter 6: VNC with the Remote Framebuffer Protocol
Chapter 7: WebSocket Security
Chapter 8: Deployment Considerations
I would like to highlight three sections of the book that I do recommend a second read.
Chapter 2: The WebSocket API - If you are not familiar with sockets you should read more than once this section. The authors have had the caution of explain each part of the API to the readers and also a amazing example of drag-in-drop image is given in the last item of this chapter.
Chapter 3: The WebSocket Protocol - Protocols are a essential part of the computers and therefore are also true for WebSockets. A brief introduction of protocols is given in this chapter and how you can inspect and understand the WebSockets protocol, also the handshake is presented. Be aware in the handshake steps because it is a hard task to accomplish it involves headers from the client and a little bit of operation with sha1, the authors chose nodejs to create a simple handshake between client and server but if you're interested in PHP you can see this post to a PHP version.
Chapter 7: WebSocket Security - As a every application the security is a point of attention. This section of the books brings to us how WebSockets work with security to prevent DDoS, Proxy Server attacks, Man-In-the-middle and so on. The book in this section has great examples and explanation about security and also shows a handshake feature in the security point of view.
Check these links to further information about WebSockets !