I have to say that it's ironic that although the book covers Windows 8.1 and is mostly applicable to Windows Phone 8.1 as well, announcements at //build changed the landscape almost immediately. For one, WinJS is now open source and isn't specific to Windows Store Apps, and WinJS 2.1 for Windows Phone has some differences from WinJS 2.0 for Windows Store apps. Oh well. Guess it's onto the third edition now! Still, I suspect that this book, being free and the most comprehensive, will continue to be the primary WinJS reference, open source or not.