Broken Symmetry, by Dan Rix, is a surprisingly engaging bit of mirror mystery.
In this world, people with the right gene can go through a mirror into a
slightly distorted version of the universe, where events are *not* reflected
into the original world. The original world (labelled "source") is where the
action starts; an unscrupulous group of researchers and teenagers with
mirror-travelling powers exploits the travelling power, ravaging parallel
universes for treasures and even replacement hands. It's dangerous work, and
more so as the government has a mysterious "artifact" that somehow relates to
travelling. The book is somewhat confusing, as the characters dart around in
slightly-variant alternate universes constantly. But it's somewhat dramatic,
as the characters work hard to finish their quest before the stress of mirror
travel destroys them. The plot twists in intriguing and unpredictable ways, and the characters have actual personalities.
The crudely-done teenage romance doesn't help though.
Two tape X's on mirrors so you can get back through them right, out of five (which is to say, worth reading if the mirror mystery appeals to you.)