A little over a year ago, I worked on the last revisions of our book "Windows Runtime via C#." At the time, you could use the WinRT API to write Windows Store apps for tablets and PCs. Now you can use the same API to target apps for Windows Phone 8.1 as well. So, Jeffrey and I went over all the chapters in the book and wrote a blog post about the differences between the two target platforms. Thanks to Jeffrey's foresight and experience, the book is general in nature and hence the differences are minimal. You can find the blog post on the MS-Press web-site here. Jeffrey also ported all the sample code to Universal apps so that you can run them on both Windows 8.1 as well as Windows Phone 8.1. Enjoy.