I read this book on my Amazon Kindle.
There was a girl whose father disappeared and she went on to later volunteer/work for some sort of research company. She had a dream where she saw a man shoot her neighbor and he turned out to also work at this company with an older man who is the boss.
They tell her it is possible to travel through mirrors but the timelines are the same until the moment you pass through. They at first play around with it, make some fake money using the mirrors, mess around with people etc. She goes to her mirror high school prom where in that universe she is popular.
Her dad comes back briefly but is "crazy" and said he had been looking for her for a long time. He either dies or gets taken away.
The girl does some mirror exploring on her own and figures out that the boss is trying to find the "surface world." The infinite worlds degenerate the further you get away from the surface because the reflection is not as clear. The boss tries to stick her in a copy world by taking her there and breaking the mirror she came through. You can only use the mirror you came in through or else you are going to a copy further down.
Anyway, the boss steals the "key to the surface" and takes off through mirrors. They track him without getting lost because he used the pattern of her DNA to make a maze of mirrors and they figured it out. They do get lost and have to back track a bit but they eventually find him with his arms cut off because he was trying to fit through the mirror. It was a mirror from the benches you see at the shoe store. She had crawled through it as a child and not realized. Her whole life she had been in a copy world and didn't know it.
She manages to get through the small mirror and find out the Dad that had come back was her actual dad that figured out where she went. And the boy who she had been working with was real and waiting for her on the surface world as he could see through his copy's eyes when he slept.