Polymyositis & Dermatomyositis Support Group
Polymyositis (PM) is a type of chronic inflammation of the muscles (inflammatory myopathy) related to dermatomyositis and inclusion body myositis. The inflammation is predominantly of the endomysium in polymyositis, whereas dermatomyositis is characterized by primarily perimysial inflammation.
And your doctor.
Also, on that same page you can request printed information to be mailed to you at no cost. I had a bunch of the brochures sent to me so that I could give to family/friends/etc.
You can also go to the mayo clinic's website:
http://mayoclinic.com/health/polymyositis/DS00334