"The Hork-Bajir Chronicles" by K. A. Applegate (Animorphs)

He is called Dak Hamee. His people believe he is a 'seer'. That he will be the one who will show them the truth.

Her name is Aldrea. Daughter of Prince Seerow. Daughter of the Andalite who was ultimately responsible for the rise of the Yeerk Empire,

Their story begins on the Hork-Bajir home world. Before the invasion. Before the terror. Before the Yeerks...

This is the story that came before Animorphs...