Is a children's book a celebrity book when Stephen Hawking writes it? I'm guessing it is if, as the BBC reports, the premise for the undertaking is the following:
"Physicist Stephen Hawking and his daughter are to write a science book for children which will be "a bit like Harry Potter", but without the magic. They aim to explain theoretical physics in an accessible way to youngsters."
"Explain" is the dead giveaway.