Resistance/Voltage relation in Rheostat?
I'm not familiar with that book, but it sounds like he is describing a potentiometer used as a volume control.

The pot would be connected like so:







simulate this circuit &ndash; Schematic created using CircuitLab

Turning the pot clockwise will move the wiper upwards.  In this application, the pot can be considered as a voltage divider.  As you turn the pot clockwise, the upper resistor of the voltage divider reduces in value, while the lower resistor increases in value, so the output voltage (and the volume) increases.