An entertaining series of books written by
K.A. Applegate.
The basic premise is that 5 kids were walking home, taking a shortcut through a construction site, whereupon they come accross a crashed spaceship, with a dying Andalite alien who gives them the power to morph into any animal once they touch it and acquire it's DNA.
Their enemy is the Yeerks, small alien parasitic slugs that crawl into a person's ear and take over their brain. By morphing various animals from the zoo and forest, they battle evil, especially the one Alien commander "Visser Three", who also has the morphing power.
The Yeerks have enslaved other planets, and have other aliens such as Hork-Bajr and Taxxon. With one Andalite cadet on the kids' side, they fight to save the planet, as there may never be reinforcements.
Each book is told in the first person narrative of one of the characters:
Jake: the leader
Rachel: the aggressive one
Marco: the comedic one, the whiner
Cassie: the animal lover/treehugger
Tobias: the quiet one, trapped in morph as a hawk
Aximili: The Andalite, younger brother of the first Andalite
There are 54 books, not counting the 1-episode parallel books. The plots became a little repetitive, more often than not ending the story with few changes to the war and few plot arcs, though eventually towards the end a long plot arc to the end of the war took place. Nickeleodeon picked it up as a TV series during SNICK, where it was criticized by fans and non-fans, done by Scholastic books.
The 54th book was the last, not including the specials and spinoff series. K.A. Applegate admitted in a reddit interview that many of the books in the middle of the series were ghostwritten, after publishing one book a month.