A subsite of the EPA's mammoth Web site, IAQ has three main sections... office, home, and school air quality. Each section contains articles on both diagnosing and solving air quality problems.
This is a very thorough online resource of radon information. As with many of our links, this site is sponsored by a for-profit company, Air Chek, that markets a very reasonably priced radon test kit. They also have a very long list of links to other radon related sites... both government and private. If you have any questions about radon, this is a great place to start!