I'm afraid I have no idea what any of the unsolved questions are, but I'm going to post mine anyway. I'm also going to be a rebel and post a book.
It was about a man who slowly faded from reality into a sort of alternate limbo reality, where everything was grey, and he couldn't touch anything. As if he were a ghost. It happened slowly, with people paying less and less attention to him, and things getting greyer and greyer.
I remember him meeting other people in the limbo; a man with an accordion (I think), and a woman. I also remember them finding the occasional item that had colour, like an orange or a towel, that they could interact with.
I read this book when I was in primary school and loved it, but I cannot for the life of me recall what it is.
Also, the book cover may have been a monsters hand holding the planet; but that makes no sense considering the story, and is most likely a false memory from something else.