The Assassins of Rome: The Roman Mysteries Series, Book 4

It is A.D. 79. In Ostia, near Rome, Flavia Gemina and her three friends-Nubia, Lupus, and Jonathan-are celebrating Jonathan's birthday. Then a visitor from the past arrives to see Jonathan's father. The next day, the visitor has disappeared- and so has Jonathan. When the three friends discover that Jonathan has gone on a secret mission to Rome, they immediately follow him. Their journey takes them to the Golden House of the Emperor Nero, where a deadly assassin is at work, and brings them face-to-face with the terrible destruction of Jerusalem....