“No theory of religious politics or religious violence in our time can possibly be complete,” Eliza Griswold writes near the beginning of “The Tenth Parallel: Dispatches From the Fault Line Between Christianity and Islam,’’ “without accounting for the four-fifths of Muslims who live outside the Middle East or for the swelling populations of evangelical Christians whose faith is bound up with their struggle for resources and survival. I wanted to go where such lives are actually led, where wars in the name of the religion are not Internet media campaigns to ‘control a global narrative’ but actual wars fought from village to village and street corner to street corner.”