Wikipedia entry on stained glass.
Stained Glass Windows History/ Glossary/Online Examples.
New York Carver. An illustrated history of medieval stained glass
There are some guilds, associations and magazines around where you can find more information on contemporary stained glass.
Artists, Crafters, Suppliers
There are many stained glass galleries, workshops, suppliers and artists the world over. I can only mention a few here - I suggest you use Google to find more.
Creative Glass Guild. (UK) Commissions, courses, materials.
Delphi Glass USA based glass supplier.