Note the above section pertaining to FreeSmart.
In The Long-lost Yoyle City, Book tries to warn Bubble and Ruby about the side effects of eating yoyleberries, but they don't care. Later, Bubble says that Book's plan to kill Pencil sounds good.
In Welcome Back, in response to Pencil falling and Bubble popping, Book says "Oh great, not again." Later, Book and Pencil recover Bubble.