I am new to windows phone programming...any good book to start..so that i can learn the essentials...or any other good website to start with???
There is Petzold's "Programming Windows Phone 7". It is currently available for free in an unfinished draft version. The finished version will also be free once released.
I've read much of the current version, and learned a lot.
Windows Phone 7 apps are Silverlight applications (or mobile XNA apps). If you are comfortable with Silverlight, the Petzold book is a good starting point. If you haven't done any Silverlight (or WPF/XAML), then I'd recommend you pick up a good starter book on general Silverlight - most of the techniques learned there will apply to Windows Phone 7 as well.
Not a book but the best place to start is the Windows Phone App Hub
There are many tutorials, code samples, official design guidelines, forums etc.
Also check out the Windows Phone 7 developer's guide at codeplex.