Beyonders # 1: A World Without Heroes by Brandon Mull

Jason falls into the hippo's pen at the local zoo and ends up in an alternate universe.  He begins a quest to find his way back to Earth, but is sidetracked by another quest to free the world of Lyrian from its evil leader.  This book is funny, very enjoyable, and has an excellent message about what it takes to be a hero.  An excellent beginning to a series!
If you are familiar with Brandon Mull's Fablehaven series, this series very different.  Yes, this one contains a little magic, but it doesn't contain demons, unicorns, or any of the other well-known magical creatures from the Fablehaven series.  Mr. Mull has created his own creatures for this series. 
I highly recommend this series!  It will probably be most enjoyed by middle school readers, but it has such a good message that I think it should be read by all!