Since there are so many of these darn things the review shall be divided into three sections. First, a brief description of the Loeb series of books and their advantages/disadvantages. Second shall be my thoughts on the author himself, his accuracy, as well as his style and the style of his translator. This is of course only my opinion and should be treated as such. The final part shall review what this particular book actually covers.
The Loeb series date back to the turn of the last century. They are designed for people with at least some knowledge of Greek or Latin. They are a sort of compromise between a straight English translation and an annotated copy of the original text. On the left page is printed the text in Greek or Latin depending on the language of the writer and on the right side is the text in English. For somebody who knows even a little Greek or Latin these texts are invaluable. You can try to read the text in the original language knowing that you can correct yourself by looking on the next page or you can read the text in translation and check the translation with the original for more detail. While some of the translations are excellent mostly they are merely serviceable since they are designed more as an aid to translation rather than a translation in themselves. Most of them follow the Greek or Latin very closely. These books are also very small, maybe just over a quarter the size of your average hardcover book. This means that you'll need to buy more than just one book to read a complete work. They are also somewhat pricey considering their size. The Loeb Collection is very large but most of the more famous works can be found in better (and cheaper) translations elsewhere. If you want to read a rarer book or read one in the original language then you can't do better than the Loeb Editions.
There are three volumes of Ammianus' surviving works. Ammianus is the Tacitus of the 4th Century. His work was originally a continuation from the time of Tacitus but only the portion from 353-378 AD has survived. His work is easy to read, generally accurate, and filled with exciting events and interesting characters. Ammianus was a career soldier who was an active participant in many of the events he describes. He knew personally many of the people who's deeds he relates. The real hero of his book is the emperor Julian. Julian the Apostate is a very sympathetic character to modern minds, and Ammianus both liked and admired him. Further sources on Julian's campaigns include his own writings (Volume I,Volume II, and Volume III), Zosimus' Historia Nova, and the remains of Eunapius in 'The Classicising Roman Historians.' Ammianus was the last great Latin historian. All of those other sources are in Greek. A better translation would probably be the Penguin one called The Later Roman Empire, although the translation here is alright. The other Loeb editions are available here and here.
This volume begins with Julian's brother Gallus and his disgraceful actions while ruling as Caesar in Antioch. It details Julian's ascension to the position of Caesar (Junior Emperor) and his campaigns against the Germans, including the famous victory at Strasbourg. On the Eastern front it details Rome's conflicts with Persia and Constantius' attempts to handle them. the highpoint of this book is undoubtedly the description of the siege of Amida. Ammianus was there and he narrates his adventures in and around the city with a great deal of personal touches. This is as close as you get to having a Roman military memoir, although the Greeks have their own in Xenophon's Anabasis.
Ammianus gives us a detailled and interesting picture of the late Roman Empire and the workings of the Roman army. The Latin text makes for great reading for anyone who has an interest in Latin and history. My complaints about this edition are that the translation does not make for easy reading, the work is divided into three, and it is very expensive. If you want the Latin text only a much cheaper alternative is Rerum gestarum libri XXXI: (ab excessu Nervae)