Normally, I'm not the sort of person who would notice things like this until they are revealed later in the film in its due course, but I got that he was the ghost as he was leaving his daughter's bedroom, when he was about to go through the door and the one last book fell as a last desperate message alongside the STAY. While it didn't ruin the film for me, it did mean I went through it with one eye trained on working out how he was going to become the ghost. It meant, for example, that the moment they mentioned the black hole, I was almost positive they'd find their way into it at some point.