Cathedral visiting became a hobby for me. Every few months I would drive to one of England's old cities, check into a hotel and study the church. This way I saw Canterbury, Salisbury, Winchester, Gloucester and Lincoln, each one unique, each with an intriguing story to tell. Most people take an hour or two to "do" a cathedral, but I like to have a couple of days.

The stones themselves reveal the construction history: stops and starts, damage and rebuilding, extensions in times of prosperity, and stained-glass tributes to the wealthy men who generally paid the bills. Another story is told by the way the church is sited in the town. Lincoln faces the castle across the street, religious and military power nose to nose. Winchester stands amid a neat grid of streets, laid out by a medieval bishop who fancied himself a town planner. Salisbury moved, in the 13th century, from a defensive hilltop site—where the ruins of the old cathedral are still visible—to an open meadow, showing that permanent peace had arrived.

But all the while a question nagged at me: Why were these churches built?

There are simple answers—for the glory of God, the vanity of bishops and so on—but those were not enough for me. The building of the medieval cathedrals is an astonishing European phenomenon. The builders had no power tools, they did not understand the mathematics of structural engineering, and they were poor: The richest of princes did not live as well as, say, a prisoner in a modern jail. Yet they put up the most beautiful buildings that have ever existed, and they built them so well that they are still here, hundreds of years later, for us to study and marvel at.

I began to read about these churches, but I found the books unsatisfactory. There was a great deal of aesthetic guff about elevations, but not much about the living buildings. Then I came across The Cathedral Builders by Jean Gimpel. Gimpel, the black sheep of a family of French art dealers, was as impatient as I with discussions about whether a clerestory "worked" aesthetically. His book was about the dirt-poor hovel dwellers who actually put up these fabulous buildings. He read the payroll records of French monasteries and took an interest in who the builders were and how much money they made. He was the first person to notice, for example, that a significant minority of the names were female. The medieval church was sexist, but women as well as men built the cathedrals.

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Another work of Gimpel's, The Medieval Machine, taught me that the Middle Ages were a time of rapid high-tech innovation, during which the power of water mills was harnessed for a wide variety of industrial applications. Soon I was taking an interest in medieval life in general. And I began to get a picture of how the building of the great cathedrals must have seemed like the right thing to do for medieval people.

The explanation is not simple. It is a little like trying to understand why 20th-century people spent so much money exploring outer space. In both cases, a whole network of influences operated: scientific curiosity, commercial interests, political rivalries and the spiritual aspirations of earthbound people. And it seems to me there was only one way to map that network: by writing a novel…

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