What series of books changes people into animals?

You're likely thinking of the "Animorphs" series by K.A. Applegate. This popular science fiction series follows a group of teenagers who can morph into animals. They use this ability to fight a secret alien invasion.

Here are some other series that feature characters transforming into animals, though not necessarily the primary focus:

* "The Chronicles of Narnia" by C.S. Lewis: While the children in this series don't physically morph into animals, they do interact with and sometimes even become part of the magical world of Narnia, where talking animals and mythical creatures exist.

* "The Spiderwick Chronicles" by Tony DiTerlizzi and Holly Black: This series involves a trio of siblings who discover a guide to the hidden world of fairies and other magical creatures, which includes some creatures that can transform into animals.

* "The Beastly" by Alex Flinn: This young adult novel tells the story of a teenage boy who is cursed to transform into a beast for a year.

Let me know if you're thinking of something else!