In the Torah periodical ישורון for Kislev 5767, in an article on "Derashat haRamban - Torat Hashem Temimah", on page 38-9 (and specifically in footnote 14) Rav Yaakov Yehuda Zilberlicht discusses what Ramban is referring to by ספר הרומאים (The Book of the Romans). Possibilities that he raises:
- It is obvious that Ramban is referring to a Roman history book, the question is: which one?
- הגר"י ספרקה (Rav Y. Spraka?) in his commentary "Chazon haGeulah on Sefer haGeula) says that this is referring to Josephus. Especially because it records a parallel event there. Also Rav Chadah in his commentary on Ramban.
- General Roman history books (and not Josephus) - since Ramban makes specific reference to Josephus in a different place, if he wanted to refer to it by name, he would have. Also, there are many other places in Ramban where he refers to "Sifrei X" - "the books of X", X being the name of other nation (Casdeans, Egyptions, Umat Tzava, etc), so there is precedent for Ramban referring to history books of some non-Jewish nation in this way.