There is a major omission on page 1 in the ninth paragraph (cited below):
You can only reverse a DC motor if you reverse the polarity of either the armature .OR. field, but .NOT. both: Shunt wound, series wound and compound wound motors DC derive their field current from the voltage applied to the armature, meaning both are reversed simultaneously, and the motor will continue to spin in the same direction.
"The good news is that as we saw in Chapter 3Ö And if we want to motor or generate with the reverse direction of rotation, all we have to do is to reverse the polarity of the armature supply. The d.c. machine is inherently a four-quadrant device, but needs a supply which can provide positive or negative voltage, and simultaneously handle either positive or negative current.