When I was building my own version, Kai Chang's RF and Microwave Wireless Systems was a really great resource for the basics. Beyond that, you'll want to have a good foundation in signal processing to make use of the data from the device. The author doesn't mention it, but the radar he has is capable of creating SAR images. I'd love to see the results of a 6 GHz radar -- Dr. Greg Charvat (one of the MIT guys) has a demo on his website of a 10 GHz version that's pretty impressive. His thesis can help with some signal processing details, but the book Spotlight Synthetic Aperture Radar: Signal Processing Algorithms by Carrara, Goodman, and Majewski is even better.
Are there any good books/resources to get up to speed on things like this?