The last known manuscript of Eusebius “Gospel questions” is mentioned by Latino Latini in a letter to Andreas Masius. The information about it begins “Sirleto wants you to know…” The quotation was printed by Angelo Mai when he first printed the remains of that work of Eusebius, reprinted by Migne, and so on. I’ve been trying to locate the letter in which Latini says this, without much luck.
But in a way, perhaps I am looking at the wrong end. Latini never saw the manuscript. His information came from Sirleto. Possibly it came by word of mouth, but equally there might be a letter somewhere from Sirleto to Latini — such letters do exist.
What I need to do, I think, is to find the correspondence of Sirleto. Do the papers of Latini contain the letters he received, I wonder? Pierre Petitmengin will know, so I ought to ask him. Have Sirleto’s letters been published?
I know nothing about Sirleto. In Petitmengin’s article I find mention of P. Paschini, Guglielmo Sirleto prima del cardinalato  in Tre ricerche sulla storia della Chiesa nel Cinquecento, Rome, 1945, p. 153-281. There is an article on Sirleto, Guglielmo in the Biographisch-bibliographisches Kirchenlexikon, 10 (1995), c. 532-33. There is Denzler, Georg. Kardinal Guglielmo Sirleto : (1514 – 1585.) Leben u. Werk. Ein Beitrag z. nachtridentin. Reform. München : Hueber, 1964. I hate books in German.
Online there is an article in a curious Catholic Encyclopedia site (a site which puts page scans online and then meanly defaces them!), and in the Italian Wikipedia, which links to a site about cardinals in English with a Sirleto article with bibliography. Looking through the last, someone published stuff from Sirleto’s papers, and suggests that these are in the Vatican.