The heir to the throne only appears in a book closely related to the Acts, the Evangelium de transitu Mariae, under the name Abdan, a short form of Abdagases, as the coins call him. Source
What kind of book is this? Have it published yet? To which century does it belong? Any further information?
It is not one kind of book but a collection of around 20 texts, all covering the life and death of the virgin Mary, but quite different from each other. It is more properly called Transitus-Mariae-literature, part of a large medieaval tradition.
Compared to other apocryphal books they are commonly dated quite late, but the exact dates or even order of dates between them is not readily apparent:
Mary Clayton: "The Transitus Mariae. The 'Tradition and its Origins", Apocrypha, Vol 10, 1999, pp. 74-98.
If the author of that lecture is correct in equating 'Abdan' being Abdagases then he seems to make quite an early connection as Abdagases as he "ruled during the first decades of the 1st century AD." Note though that this is a stretch and even if correct it only shows a literary connection, in themes, not a date of these texts.
Well, we can get the source reference explicit: The Schweich Lectures on Biblical Archeology 1934: Archeological History of Iran, by Ernst E. Herzfeld, DThil, published for the British Academy by Oxford University Press in 1935.
Ernst Herzfeld was a professor at the Friedrich-Wilhelm University of Berlin and a notable archeologist, specialising in Iran. The Schweich Lectures have been given annually at the British Academy since 1908.
The Acts of St Thomas is a piece of New Testament apocrypha, telling the story of Judas Thomas' missions to India. The reference seems to be to the text now known as The Passing of Mary.
The book is The Assumption of Mary is an apocryphal book available in quite a few versions. The CCEL editor in the introduction dates the Greek text he uses to "no later than the fourth century" and the Latin versions are not dated but said to be adaptations of it.
Text at CCEL (English): Greek Version, Latin Version 1, Latin Version 2
The same book is also available at archive.org.
More bibliographic information is at e-Clavis, which also includes links and references to translations and ancient versions in other languages.
- Assumption of the Virgin (both Latin versions)
- Dormition of the Virgin Transitus Greek R
- Dormition of the Virgin by the Apostle John
However, I see no reference (at least in the versions translated to English) to any Abdan as the successor to Gundopharr as king of India, as the book claims. Since the book is said to be closely related to the Acts of Thomas, maybe the name appears in one of the Syriac versions.