Brandon Mull's Books
Brandon Mull is an author that has written three of my favorite series. Fablehaven, Five Kingdoms, and Beyonders. He has also written the Candy Shop War books and Pingo.
My personal choice of these is Fablehaven, which has a followup series called Dragonwatch. See my post about Fablehaven here.
5K and Beyonders are tied together, with many of the same characters.
As far as I can tell, Beyonders is set a few years before Five Kingdoms. Beyonders follows the adventures of two children from our world, Rachel and Jason, who accidentally travel to another dimension (by following a rare butterfly and being eaten by a hippopotamus, respectively). There is much character development as the series goes on, which is why I like it, along with an awesome twist ending.
Five Kingdoms follows the resulting catastrophe after a boy gets involved in inter-dimensional slave trade. If I say any more I will spoil it.
Read on, friends!