The second part of the Circle trilogy from Ted Dekker. This one definitely got me hooked right from the beginning and was well worth the read.
Red centers around the heroic rescue attempts of Thomas Hunter to save both worlds. With the devastating realization that he has unleashed death into both worlds, he now must figure out how to undo this evil all while trying to save his true love. From meeting with world powers in one world to leading a small band of mighty warriors in the great forests of the other world, Thomas is determined to change the history of two realities, or sacrifice all in trying.