Alternate historian Turtledove (A Different Flesh) gets heavy-handed with this allegorical novel that swaps clichéd ideas of Europe and the Islamic world. Senior Investigator Khalid al-Zarzisi, a Muslim, and his partner, Investigator Dawud ibn Musa, a Jew, have flown from Tunis to Rome to assist Grand Duke Cosimo in dealing with the increasingly dangerous Aquinist fanatics. The Aquinists take to an extreme the general European distrust of science and devaluation of women, unlike the very liberal and technologically superior Muslim republics. While Khalid and Dawud are well-drawn characters, with Dawud having a delightful sense of humor, the two are more observers than investigators. They visit a few hot spots of Aquinist activity, get involved in a few firefights, and make cogent suggestions to the grand duke and the pope, but are often simply witnesses or commentators. Readers are left with the ongoing, barely nuanced comparison between the modern, enlightened Muslims and the backward Christians, and the inference that actual history and politics can be just as easily simplified. Turtledove's characters provide some interest, but this take is generally unsophisticated and unsatisfying. (Sept.)