As a child I read a book that describes 7 layers of existence, 'we' are living in the 3rd one and we know heaven as the 5th. The book describes a journey, I believe by a boy and his sister and possible some more, of them travelling from the 3rd through the 4th to the 5th world.
I don't know much about it anymore, it's easily been 20 years. I seem to remember it discussed a nazi concentration camp as well. Does this sound familiar to anyone?
Update: I'm also quite sure either the 1st or 2nd layers were the ones for stones and other inanimate objects. If it was the first, the 2nd may have been plantlife but that's just a guess.