We are about half way thru the book and Xamarin just released the next new version: Xamarin 3 and it's awesome. One of the new things in Xamarin.Forms that is a completely cross platform set of UI controls. It sort of invalidates the second half of the book. I think we should take the concept of a chat app expressed in the book and try implementing it using Xamarin.Forms. We could even release it as an app in the store.
check out the google group for more details:
Xamarin Cross-platform Application Development
read a chapter a week and we'll compare notes at the meetings. We'll also spend time messing with hardware and deploying to iOS and Android devices.