I used to read a series of children's 9-12 or young adult aged books, the last time I read this book was at least 15 years ago when I was about 12 years old.
It was about a group of 4 children (2 boys and 2 girls), one of whom was named Adam (perhaps there was a Sally also?). All I remember, was that these children used to walk backwards in a graveyard, and they would find themselves in a fantasy world with demons and fairies; it was a very different world each time. One book, they found themselves in a castle and they find 4 gems and they are given different hyper qualities. One of the boys becomes very strong, and one of the girls (this I remember more vividly) becomes fair/beautiful until she becomes so white that she literally begins to fade away like a ghost. I believe that the graveyard exists in every reality/world that they travel to. It seems to be that they need to pass through there to get home every time.
One of the books had a yellow front cover, with a vortex multicoloured spiral in the centre.
I used to borrow this book from my local library in the children's section which was not too big, I have a small inkling that the author of this book had a last name from E-S from picturing the layout of the library (but I may be wrong).
I don't remember this book being particularly famous, I don't think I have seen it since.