The documentation for the Window Phone Developer Tools (WPDT) is now available for viewing offline and is accessible from within the Visual Studio. Installing Offline Documentation
1. In Visual Studio 2010 or Visual Studio 2010 Express for Windows Phone, click on Help > Manage Help Settings.
4. Click Update. After installing the documentation you can access it by going to Help > View Help.
If you are directed to the online version, you will need to change the setting to point to offline docs in the Help Library Manager by clicking on Choose online or local help.