First, my newly-released second edition, http://aka.ms/BrockschmidtBook2, is very applicable to Phone apps. I didn't have time to spell out all the differences, but there is a summary of what to watch out for in Chapter 1, namely a few differences in controls and some parts of WinRT that aren't on the Phone.
Second, watch the Building Apps for Windows Blog for material in this area. For example, an upcoming post will spell out the controls story between Windows (WinJS 2.0) and Windows Phone (WinJS 2.1).
Finally, there is a post on the Visual Studio blog on universal apps: http://blogs.msdn.com/b/visualstudio/archive/2014/04/08/building-windows-phone-8-1-apps-in-html.aspx.
More is certainly to come.