Animorphs is a series of science fiction books for children written by K.A. Applegate and published by Scholastic from 1996 – 2001. The six main characters (five humans and one alien) have the ability to ‘morph’ into any animal they touch. Using this ability they battle a (secret) infiltration on earth from the alien species Yeerks.

The series consists of 54 books with 12 companion novels:
Megamorphs #1-4
Elfangor's Journey, Alloran's Choice, An Alien Dies (also published in 1 volume as ‘The Adalite Chronicles’)
The Hork-Bajir Chronicles
The Ellimist Chronicles
Alternamorphs #1 & 2.

Currently out of print, this series is popular with young readers today (ages 10+), with some of the later titles in the series becoming increasingly harder to source second hand.