For anyone looking for a great book on developing Windows Phone 7 applications, take a look here.
“This book combines useful, technical information about the platform, written by those who’ve been there and done it, with some stories about the people and the apps they’d written. The book will act as a handy reference that gives you a ‘puddy up’ into developing on the platform and, importantly, helps you to avoid making the same mistakes others have made.”
The best news is that the digital download is completely free, although you can also get a hard copy for a mere £3.86!