Animorphs...

Animorphs is an English written sci-fi series aimed for young adults. Published from 1996 to 2001 and consists of 54 books and 10 companion books.

"Five humans, one of whom eventually becomes a hawk, and one alien obtain the ability to morph into any animal they touch; they name themselves "Animorphs", a portmanteau of "animal morphers". Using their ability, they battle a secret alien infiltration of Earth. It is told in first person, with a different narrator in every book."

I find the concept of the animal morphs quite compelling. It would be amazing to even just imagine what experience they go through to turn into the animal they touch. What kind of things they feel, see, sense and hear, and how restricting or free would it be to be trapped inside an animal.

http://en.wikipedia.org/wiki/Animorphs