Cousins Per and Radius hunt for treasure by unraveling a Viking mapmaker's clues: a series of coordinates (x, y) that pinpoint locations on the Viking's map grid. Highway robbers, channeling the Keystone Kops, chase the cousins around, but this subplot comes off as rather superfluous. Acrylic paintings with a medieval setting illustrate this introduction to coordinate (a.k.a. Cartesian) planes.

In the seventh addition to the Sir Cumference series, cousins Per and Radius are lost in the medieval countryside when they discover a map decorated with two hand -axes- featuring X and Y coordinates. The map promises to lead them to treasure belonging to Viking Xaxon Yellowbearyd. As Per and Radius follow the map on horseback, they discover additional X and Y coordinate clues left by Xaxon, which readers can help them locate on the large map. Outwitting a gang of bandits, Per and Radius follow the last coordinates to Xaxon-s ghost, who gives them the treasure (in the form of more maps). Neuschwander does an admirable job of injecting humor (namely, some groan-inducing puns) and action (bandits! ghosts!) into this explanation of coordinates and axes, and Geehan-s thickly worked paintings contribute some drama of their own. Ages 8-11. (Feb.)