What's a flying buttress?
In the 1100s AD, architects in northern France wanted to build big impressive Gothic cathedrals, and they wanted their cathedrals to be full of light, so they would be inspiring, not dark and depressing. But if the walls were mostly glass windows, how would they hold up the heavy stone roof?
The flying buttress would start from the places at the top of the wall where the groin vaults were directing the weight of the roof. From there, the flying buttresses would carry the weight of the roof away from the building and down a column of stone to the ground. It wouldn't matter what the walls were made of anymore, because they wouldn't be carrying the weight of the roof.
More flying buttresses (Rouen 1200s AD)
Once architects began to use flying buttresses in their churches, they began to make more and more of the wall out of glass, and cathedrals looked lighter and more heavenly. They used flying buttresses at Notre Dame and the Sainte Chapelle in Paris, at Chartres, at Rouen, Reims, and Amiens cathedrals. Architects also used flying buttresses in England, at Westminster Abbey. The flying buttress did not catch on so much in Italy, where people liked their cathedrals to have a lot of stone and not so much glass.
Arches to Zigzags: An Architecture ABC, by Michael J. Crosbie (2000). Shows what an arch is, or a gable, or an eave. For younger kids.
Eyewitness: Building, by Philip Wilkinson, Dave King, and Geoff Dann (2000). Lavishly illustrated, like other Eyewitness books for kids, and with good explanations of most architectural terms.
City: A Story of Roman Planning and Construction, by David Macaulay (1983).