I thought this was a lovely book, and appreciate Oliver Jeffers's ability to let the reader create their own story out of this simply told, and beautifully illustrated, tale.
It's not really clear if the chair is empty because the man died, or if he has just gone away for a while. Is he even the grandfather? My six year old interpreted the story her own way, that the man went on vacation, and didn't find the story sad at all.
I met Oliver Jeffers a few weeks ago, and asked him about this book in particular, and his answer was that he left it up to the reader to find their own story, which I like very much.
I definitely recommend this book, and all others by this amazing author/illustrator.