Indeed, the book was delayed a little. However, it is ready now - and the title changed to "Professional C# 6 and .NET Core 1.0".
With the VS2013 edition you mean there haven't been that many changes compared to the VS2012 edition? That's true.
This is very different with the new version. You're right in expecting a lot of changes and additions with C# 6, .NET Core 1.0, and Visual Studio 2015.
It was not easy with the late changes of .NET Core, which also delayed .NET Core. I incorporated the updates into the book; it also covers the new .NET Core command line (CLI) tools and other new features.
The code files on the Wrox site are currently based on RC1, but the book shows newer code, and I don't expect that the code itself changes with RTM a lot. When RC2 is available, the code download on the Wrox site will be updated. In the meantime, you can also get and read the code samples at http://www.github.com/ProfessionalCS...ssionalCSharp6
I hope you enjoy the new edition!