Twice Upon A Time by John Pinkney is an Australian book published in 1996.
I think someone must have misjudged my reading abilities because inside the book I've written in pencil '14th birthday' which is what I often did when I got books as gifts. I think it was a little young for me, but I still enjoyed it!
From the back cover:
Sara wakes- to find herself wandering through the dark maze of cellars beneath St Ann's College. She is drawn to an old wooden door whose cracks emit a dim, flickering light.
Behind that door lies a secret, a secret that will change Sara's life. Forever.
This book seems to be quite rare now, it's only on Amazon as second hand (for 27 pounds I might add!) so I'm not sure if it was ever 'popular'. However, since it was in my shelves, it did have an effect on my life. It was right up my alley.
Sara and her little sister Ella are orphaned and living in a boarding school since their grandmother is too frail to look after them. The junior school is moving to a new campus, meaning Ella and Sara will be separated. I found this a pretty devastating concept as a 14 year old. :( And not only that, but the woman in charge of the junior school is a horrible woman who won't let the girls visit.
The secret? Stop reading now if you don't want to know...
Sara finds a door in the cellar of the school while sleep walking, a door that takes her to another dimension where there's another version of herself. She can meld into that other version's mind/body and they become one. The only thing is, the dimension changes each time she goes through it, from ultra happy sunshiney places to what seems to be post apocalyptic nightmares.
It stuck with me because I love the concept of alternative dimensions and they're so rarely portrayed in children's fiction. The emotions were very strong too.
If you have kids, grab a copy for them if you see it.