The Hork-Bajir Chronicles
Note! This book takes place before book #23, The Pretender, although the events told in the story occur between the time of The Ellimist Chronicles and The Andalite Chronicles.
Before the Animorphs... on another world... the fight began. Prior to their invasion of Earth, the Yeerks attacked a gentle and docile species known as the Hork-Bajir. This is the story of a special Hork-Bajir, his Andalite friend, and their future enemy, Visser Three.
K. A. Applegate
Katherine Alice Applegate (born 1956) is an American author. Applegate's most popular books are science fiction, fantasy, and adventure novels. She has won the Best New Children's Book Series Award in 1997 in Publishers Weekly.
She also writes books for young readers as Katherine Applegate.
Animorphs
Sometimes weird things happen to people. Ask Jake. He may tell you about the night he and his friends saw the strange light in the sky. He may even tell you about what happened when they realized the "light" was only a plane – from another planet.
Here's where Jake's story gets a little weird. He and his friends are told the human race is under attack, but they're given the chance to fight back.
Now Jake, Rachel, Cassie, Tobias and Marco have the power to morph into any animal they choose. And they must use that power to outsmart an evil that is greater than anything the world has ever seen...
Animorphs consists of fifty-four primary books, and includes four additional books that complement the series but are not considered mandatory reads. The current recommended reading order for the series is provided below.
Main series Animorphs Series