Thanks to Werner Meyer for directing me to this article in National Geographic on the Apostles, which goes into the research I used for Stone of Fire (originally published as Pentecost), the first in the ARKANE series.
It covers Thomas in Kerala, India; Mark in Coptic Egypt and then Venice and goes into the study of relics, which is also a theme in Prophecy.The article also talks about Mary Magdalene, who I didn't include in my list of “official” apostles, despite the popularity of The Da Vinci Code!
“To study the lives of the Apostles is a bit like what we've been doing with the Hubble telescope—getting as close as we can to seeing these earliest galaxies. This was the big bang moment for Christianity, with the Apostles blasting out of Jerusalem and scattering across the known world.”