If you are currently developing or you are planning to develop applications for Windows Phone 7, you should read this guide.
If you read version 1 of this guide, here are the highlights for this release:
1. Completely redesigned format to enhance presentation, readability, and highlight guidance more prominently
- Layout, from left to right, on every page: image, general information in normal type, guidance in bold type
- Use of color highlights per section
- More visible page numbers
- Formatted to support a hard copy version for RTM
2. Added new content to support beta release of the Windows Phone Developer Tools
3. Reorganized content into four sections: Metro design philosophy, overview of phone UI and features, application interface controls, and miscellaneous
You can download the guide from here.