While there are many photos of medieval glass, most sources on medieval stained glass are not useful for the home hobbyist. Most of the good sources, useful or not, are articles accessible via JSTOR. Since digging through these articles for useful information for the home hobbyist is like looking for a needle in a haystack, I shall present my needle-case here.
Most of my sources are cathedral windows themselves. The oldest in situ are the Augsburg Cathedral in Bavaria. While most windows from the height of medieval stained glass artistry feature single-color glass only, this was not the case earlier in the period. As windows, there is a VERY good reason for this - single color glass lets light through in a far more spectacular fashion than dichroic glass. I have yet to find any examples of iridized glass in period, nor reference to a method for doing so - I will continue with the supposition that iridized glass is wildly OOP. But dichroic glass is demonstrably found in period, though uncommon in surviving windows.
NINTH-CENTURY WINDOW GLASS FROM THE MONASTERY OF SAN VINCENZO AL VOLTURNO (MOLISE, ITALY)
Francesca Dell'Acqua
Journal of Glass Studies Vol. 39 (1997), pp. 33-41 (9 pages)
While the glass is not in situ, this article details glass that was used for windows. The glass DEFINITELY includes dichroic glass, with clear, red, and green swirled glass among the finds.
One roadblock in re-creating medieval glass is scale. One window, for example, is ~440 square feet of glass, divided into panels about 3x6 ft, if my calculations are correct (I haven't had THAT much coffee yet). I have space to make about... 4 sq ft at a time, and the reinforcement knowledge to make 14x22 inch panels. I have difficulty wrapping my head around the physical realities of making such large panels (I had a hard time moving an 18x24 inch panel), and am still trying to figure out a way to make it make sense.
Some articles, while illuminating, do not contribute to making stained glass. In 1975, a study was conducted on the weathering of stained glass, comparing Roman-era glass to medieval-era glass, which I hoped would be useful in understanding leading. The study, however, covered pitting and silica loss from surfaces as a result of environmental differentiations. While fascinating, it is not useful to a hobbyist.
Another article, about narratives in stained glass, discussed how to "read" stained glass based on position and what the positioning reflected about cultural concepts at the time. IF I were constructing massive cathedral windows, this would be useful, but as someone with a 2x2' working area for leading, it's merely background information (I would have to join at least 5 panels to use the information involved.)
Painting on Light is the most period-appropriate source, but is underwhelming in terms of practicality to a beginning glass painter. While I HAVE read these books cover to cover, I cannot cite anything I specifically learned from them, making my glass library rather underwhelming.
On Divers Arts, attributed to Theophilus, has a decent section on glass work, including directions on building a kiln for glass staining. It is one of my overall favorite sources.