Children's book in which there seems to be another person on the opposite side of their mirror I remember reading a book in which there seems to be another person on the opposite side of their mirror. Its a children's book, and it is one of the books of a big series. I can't seem to recollect which book it is. Could someone help me?

It is Zack Files by Dan Green. This is not an individual book but a series of books. The book is Through The Medicine Cabinet