Empty oceans mark most of the path of the northern 10th parallel, or latitude, as it encircles the globe. Over land, though, it divides some of the unhappiest places on Earth. From the West it carves through Nigeria, Ethiopia and Somalia. In the east, it passes north of Indonesia and Malaysia, then bisects the Philippines.
These are places where the divisions are religious rather than geographical; where Islam and Christianity are balanced in a frequently bloody struggle for hearts and minds. Part travelogue, mostly a diary of despair, Eliza Griswold's The Tenth Parallel is an account of her seven-year journey among these divided peoples.
The daughter of an American Episcopalian bishop, Griswold nevertheless displays a controlled neutrality as she attempts to unravel conflicts that have already claimed hundreds of thousands of lives and look likely to do so for generations to come.
Depressingly, the most strident and intolerant voices along this fault line belong to the extremes; violent jihadis on one side; on the other, Evangelical Christian missionaries. It is hard not to conclude, though, that the underlying cause of so much misery is not the words of the Quran or the Bible, but poverty, disease, ignorance and intolerance.