Written just above exercise #42:
"Notice how the bass is not always playing the root when the chords change. You can play other notes that belong to the chord (This will be explained in depth in Book 2)."
This happens a few times throughout the exercise, i.e., in the 2nd measure, the chord is D but it's notated for you to play an A.
In both cases, you're playing the 5th of the chord. As Ed wrote above, you'll get to that later in the book.
