I don't know what it was, but I did not like this book. At all.
The writing was strange; everything happens very fast and there's very little explanation given for a lot of the things that go on. Will, the protagonist, seems to obtain knowledge from seemingly nowhere. Everything moves at a very brisk pace (almost alarmingly so) and while I wouldn't say I was lost, it was definitely difficult finding even ground. The time-travel aspect of it all felt out of place, and Will seemed to do so with very little transition (once in awhile he walked through a door, which was the only indication, and this only happened maybe once or twice.) I couldn't bring myself to care about any of the characters. They all felt rather flat, or just didn't have any distinguishing features for me to differentiate them from the rest of the cast. The theme of the novel is a very typical good vs. evil type deal, with Evil being a disembodied force (made me think of the Big Baddy in the seventh season of Buffy the Vampire Slayer, but less cool) that also kind of bored me.
To be fair though, there is a lot of myth and background throughout the story, if you can be bothered to dig for it. I do give props to Cooper for that. Other than that though, I wouldn't recommend this book. It bored me to tears. :(