https://play.google.com/store/books/details/Charles_Petzold_Creating_Mobile_Apps_with_Xamarin_?id=C86iBAAAQBAJ
This book is for free.
From Microsoft Press.
You can download it as Google Play Book or as pdf/epub formats (with Adobe DRM protection).
Have a nice day.
P.S. I wonder why is there no mention of it on Xamarin Blog page ? Maybe C.P. will announce his book on Xamarin Evolve for 2 days officially ...