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 aren't that new, and can get sort of repetitive, sometimes more of an emotional struggle than fighting bad guys, but the kids like it. Nickeleodeon picked it up as a TV series during SNICK, where it was a pretty good adaptation, done by Scholastic books.

Oh, and I'm not sure how recently, but the 54th book was the last, after publishing one book a month.