What is an Aisle?
Usually people stood mainly in the nave, and they used the aisles for walking around, so they wouldn't disturb the people who were participating in the Mass. Sometimes there were people selling snacks or souvenirs in the aisles, too. On a holiday if the church was very crowded, people would stand in the aisles, too.
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).