Most people in the business know of this bookstore, but here it is anyway -
The best resource for used and out of print stained glass related books is Whitehouse Books in Corning NY. Corning is, of course, a glass company town so the selection has even more glass books than stained glass books. They don't have everything but they have more than any other single resource.
and if you must (as I have mixed feelings about the idea of supporting a megalithic online bookstore), I have had good luck with searching for used books through Amazon. Recently, I got a copy of Narcissus Quagliata's From Mind to Light for $12, including shipping!