I believe that I first discovered the Animorphs series, written by K.A. Applegate, when I was in 4th grade. After a couple years of reading Babysitters Club and the like, I cautiously gave this weird looking science fiction book a chance AND MY LIFE WAS NEVER THE SAME. I thank this series for turning me on to science fiction/fantasy books and making me feel like it was perfectly fine to like them and other non science fiction at the same time.
The whole concept of Animorphs blew my mind. Teenagers who could morph into an animal after touching it once? Only 2 hours before you are stuck as that animal forever? Parasites? Aliens? I was immediately sucked in. I loved the diverse group of kids: Jake, Cassie, Rachel, Tobias, and Marco. I had a little crush on Jake and thought Rachel was a bad ass. Each book had a different narrator, which I loved as well. Oh and Ax! Can't forget about our alien friend!
When I was in 4th and 5th grade, there were three other kids in my class that also loved Animorphs, two boys and another girl. We would constantly talk about the books at school and one of the boy's mom always bought him the newest book, so he would read it and then pass it around to the rest of us. Of course, everyone read it within one or two nights. I remember the four of us trying to figure out when Jake and Cassie would finally kiss (you know, that 13 year old sexual tension). I can't even imagine overhearing that awkward conversation.
Nickelodeon tried doing a TV show version of Animorphs; it sucked. I try not to let it get in the way of remembering these awesome books. I still have a bunch of them back home and plan to pass them on to my hypothetical future children (who will be nerdy and awesome). Hopefully they will spend some time bemoaning the fact that they can't morph into animals, just like I did.