One of the reviewers is quite discouraged by the overuse of the Deus Ex Machina/Diabolus Ex Machina tropes:

We start out in an alternate reality which is, well, alternate, but seems pretty reasonable. Then, all of a sudden, surprise! People can travel through time! Okay, alternate reality with time travel. Then, all of a sudden, surprise! You can go into a book! Well, that wasn't much of a surprise, since I'd read the back cover, so we'll let it go. Then, all of a sudden, surprise! There are werewolves and vampires in this world! Oooo... kay. Then, all of a sudden, surprise! The characters go through a black hole! At that point I just rolled my eyes and stopped caring-- if the whole thing was completely random, then there was basically no point to the book. No matter what mess the author got the protagonist into, he could just come up with a "oh yeah, I didn't tell you this before but they can teleport from place to place by wiggling their nose!" type of thing to save the day.