Jonah installs the day's road book into the reader. It's a scroll, and the reader is a motorized deal for scrolling the paper through it. The road book shows mileage and a diagram of the intersection or other features as well as notes on the spot. The riders take notes on the paper scroll, sometimes extensively. The road book is given to the competitors the night before each stage.
Rally-raid ain't just about going fast. To do well at all you have to be a great navigator, and Jonah is particularly good at it.