I'm sure there are many secondary sources on the 3 magi, but you can
probably get at most of them quickly by looking at R. E. McNally, "The
Three Holy Kings in Early Irish Latin Writing," in Kyriakon. Festschrift
Johannes Quasten, ed. P. Granfield and J. Jungmann, 2 vols. (Muenster,
1970), II, 667-90. Didn't the later medieval legend of the three kings
center on Cologne, and isn't there a book on this subject?