Why Double Aisles?
Very big churches needed double aisles. The extra row of columns helped to hold up the heavy stone roof. This also made room for more activities inside the church. Because medieval churches were the biggest indoor spaces in their towns, people didn't only use them for religious services. Churches were also used for weddings (as they are today), graduations, big parties, town meetings, concerts, criminal trials, and sometimes even farmer's markets.
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).