This week the beta2 version of OpenLayers3 has been released (see the OpenLayers3 is comming from Boundlessgeo blog) with great improvements on vector layers rendering and image manipulation.
Similarly, The book of OpenLayers3 follows a similar progress, always one or two steps after the project evolution.
Currently, you can found a sample chapter about the Map and View, both theory and practice, and I have almost finished the next chapter focused on how to work with Layers.
After explaining layers the natural flow is explain the Sources and Formats, which allows to load content from a great number of data sources. They are very related to layers, but I preferred to differentiate the concepts in two separated chapters. This is what I'm currently working on.
What else requires to be in the book? I need to make a chapter focused on vector data, explain how to work with geometries, features and styles. Events are also important, we can listen for changes in objects and react accordingly.
As you can see there are many work to be done yet and your feedback is the most valuable thing I can get at this moment.