June Books 11) The Provinces of the Roman Empire, by Theodor Mommsen
I bought this three years ago, ages before I had my plan of reading Gibbon from start to finish, but at a time when I had vague thoughts of another project which in the end I gave up on before I started - reading books by winners of the Nobel Prize for Literature. One of the reasons I gave up, of course, was that in its earliest phase the prize often went to worthy but unexciting writers. The second laureate, after French poet Sully Prudhomme, was German historian Theodor Mommsen, and his History of Rome was particularly cited by the committee. The Provinces of the Roman Empire, from Caesar to Diocletian was one of the later volumes in Mommsen's series (or possibly two, depending how you count them), surveying everywhere other than central and southern Italy, starting with northern Italy (including the Adriatic) and then going clockwise around the borders starting with Spain and ending with Mauretania (modern Morocco).
One of my increasing frustrations with Gibbon (as chronicled on reading_gibbon) is his habitual haziness about the geography of places that don't interest him, which is basically everywhere outside the very narrow triangle connecting Rome, Paris and London (by my reckoning Lausanne is just on the longest edge of the three). Mommsen totally reverses this, with a detailed discussion of the niceties of local administration from Scotland to Mesopotamia. I hadn't really appreciated how varied the arrangements were between client kingdoms / statelets and formal provinces. Another point that is clear from Mommsen, but denied by Gibbon in the face of the evidence, is that the Empire's boundaries were actually fairly fluid where geography allowed; central Germany, lowland Scotland, Dacia, and the eastern frontier of Mesopotamia and Armenia all slipped in and out of Roman control over the years. (In fairness, Gibbon rules out the earlier and more volatile half of Mommsen's time period by starting in the mid-second century, but I think his line that the borders were stable is still simply incorrect.)
Mommsen is a bit romantic about the Germans; gives a great account of Trajan's column; is utterly fascinated by the Jews (apparently he was an anti-Semite as regards contemporary public policy in Germany, but it doesn't really come across in his historical analysis - see comment from selenak below), very good on Palmyra, Persia and Ethiopia; very bigoted on Africa (ie Tunisia/Algeria) and Syria. The English translation did not strike me as having the excellent fluency of style attributed to the German original, let alone being anywhere near a match for Gibbon. But it filled in a lot of gaps in my knowledge, and I'm a little regretful that I didn't grind through this before starting The Decline and Fall.