Pierre Manent began working on his slim book about Islam and French society in the aftermath of the January 2015 murder of 11 people in and near the office of the magazine Charlie Hebdo. “Situation de la France” was published on Oct. 1. Six weeks later, terrorists affiliated with Islamic State murdered 130 people and seriously injured nearly 100 more in a series of attacks in and around Paris. Terrorist attacks have become more brazen during the intervening months, both in Europe and America, and Mr. Manent’s thoughtful and provocative argument—now translated into English as “Beyond Radical Secularism”—deserves as much...