"The U.S. Environmental Protection Agency has numerous publications covering various aspects of radon and indoor air quality. For a list of these publications, please visit http://www.epa.gov/iaq/pubs/index.html"
The EPA stopped operating its National Radon Proficiency Program (RPP) on October 1, 1998. That program was designed to test radon contractors and provide a measure of quality control. The RPP is now privately run. See the EPA's Web site regarding the RPP for information on how to identify qualified contractors.
Many states certify or license radon contractors. Call your state radon office for information about qualified service providers in your state.
If you plan to fix the problem in your home yourself, you should first contact your state radon office for EPA's technical guide, "Radon Reduction Techniques for Detached Houses."