This article from Wikipedia provides a simple summary on the aortic valve.

This article from Wikipedia provides more specific information on aortic stenosis.

A very nice summary from the British Cardiac Patients Association can be seen here.

A more scientific review of aortic stenosis from emedicine.com can be found here.

A published review article in medical literature on aortic stenosis can be found here. 

A useful resource for patients with aortic stenosis can be found here.