[Romulus]

The non-canonical Acts of Peter and Acts of Peter and Paul also are quite early and locate both apostles in Rome at the time of their martyrdoms. Ancient traditions, such as the crucifixion of Peter and the "Domine quo vadis" incident appear here as well. Consistent with historiographical standards and practices of their day, these documents are surely entitled to a respectful reading.

[eastsider]

I believe that the earliest "historical" (in the modern sense of the word) work to which Eusebius had access was Hegesippus' history of the Church at Rome. As I recall, Hegesippus was a second-century Syrian Christian who travelled to Rome specifically to gather whatever information he could while some of the Christians from the apostolic age were still living. His multi-volume work of the early history of Rome is no longer extant, and his work is known today only from derivative sources like Eusebius.