Java WebSocket API Handbook

The book is what it says it is - a handbook, a quick reference, a fast track guide. It covers the nitty gritty of the Java WebSocket API which is a standard (specification) for building WebSocket applications. It is targeted (primarily) towards Java/Java EE developers and can be used in various capacities
As a getting started with the Java WebSocket API guide
You are already well versed with this API and its constructs, but need a quick peek/reference to a specific API, it's usage, nuances etc.
Maybe you're just curious about what Java has to offer in terms of WebSocket support - feel free to check this out