The Loeb edition bears the inside title after the 50pp of introductory material Ammiani Marcellini Rerum Gestarum libri qui supersunt The volume titles are merely Ammianus Marcellinus although they include no other work by him since none has survived This inside title translates as The books that survive of the Deeds Acts History of Ammian Marcellinus Nowhere have I been able to find where this Latin title comes from I can t remember reading or hearing anyone actually refer to AM s History as Res Gestae although that s a Latin phrase that can be translated history The Divi Augustae sic on the other hand is a much clearer mistake which should have been so to speak Divi Augusti although FWIW not very much one can find RG Divi Augustae online Still it s not right and anyhow RGDA always refers to the piece of Augustan propaganda I ll revert the Loeb Reference although my edition doesn t have the same ISBN numbers prolly better to leave those off