Pears, author of the Restoration-era mystery An Instance of the Fingerpost, again uses the past as a canvas, but this time the history is pedantic and the only mystery concerns the apparent disappearance of his editor. The book is set in Provence during three periods of upheaval: In the waning days of the Roman Empire, Manlius Hippomanes accepts a bishopric, feeling that he's the best man to thwart the advance of barbarian tribes. In the 1300s, a courtier unearths the bishop's treatise on Neoplatonic philosophy. In the 1940s, a scholar of Manlius attempts a minor subversion of Nazi rule by accepting work as a censor. Throughout, dialogue and narration remain stiffly academic; it's fairly typical that, in speaking of love, a character remarks, ''the physical craving is a reflection of the desire of the soul to reunite with the ultimate beauty, with God.'' This is how ''the novel of ideas'' became a dirty phrase.