The Roman Emperors; The Twelve Caesars; The Fall of the Roman Empire, all by Michael Grant (Phoenix Giant, each £12.99 in UK)

These are works of popularisation rather than of academic history, though Michael Grant has good scholarly credentials. The first volume is perhaps the most substantial of the three, dealing with such remarkable figures as Marcus Aurelius the "philosopher emperor", Trajan, Hadrian, Septimius Severus and his rather repulsive son Caracalla, among many more. The second volume covers familiar territory, while the last is frankly popular in approach. The "fall" of course refers to the Western part of the empire; in the East, Byzantium/Constantinople endured for almost another millennium.