Check out the MSDN Magazine article by Microsoft Press author, Andrew Whitechapel, on building an app for Windows Phone and iOS.
MSDN Magazine article: http://msdn.microsoft.com/en-us/magazine/jj658972.aspx
There’s also an interesting follow up to his article here: http://blogs.msdn.com/b/msdnmagazine/archive/2012/10/19/10361207.aspx
Right now, you can order the ebook version of the upcoming book, Windows Phone 8 Development Internals, on The Microsoft Press Store: https://www.microsoftpressstore.com/store/windows-phone-8-development-internals-9780735676237.
You would think Andrew would have used MonoTouch for the iOS version and shared code between both versions. Here's a much better cross-platform article covering Windows Phone, iOS and Android and sharing the bulk of the code between them.