Free ebook: Programming Windows Phone 7, by Charles Petzold
Microsoft has been pushing hard to have developers on their side, firstly by releasing the free development tools, secondly by providing a strong baseline of training (some examples here and here) and also by offering devices on conferences such as PDC10.
Now another effort comes together as Charles Petzold wraps up his book and Microsoft Press makes it available for free on pdf, including its source code.
So my suggestion to you, if you are a developer, is grab the Windows Phone 7 wave because it sure is shaping up to be a contender to the top smartphone platforms in years to come.
Go on and grab your free ebook here.