I discovered Sad Book about three years ago. It is a brilliant book and it really does stay with you. I think every home should have a copy - to help when it is necessary to open difficult conversations and also just as an antedote to the saccharin sweetness that can plague children's fiction... MPD
I love Michael Rosen's Sad Book - I recommend it all the time at my library to help people talk about grief. There aren't many books which help small children to talk about death and grieving, especially not many which acknowledge that losing someone can make grown-ups very very sad, there are a few nonfiction about losing a parent/grandparent/sibling but I find them rather dry and unappealing - Sad Book is beautifully illustrated and tenderly written.
Another book I like very much (but it much simpler and is about a cat, not a person) is 'Goodnight Mog' where the cat dies, but it does feature a kind of afterlife which is a message not everyone wants to give their kids - Sad Book doesn't try to guess what happens next, he just talks about how rubbish it feels for the people left behind, and how sometimes you smile and that's fine too.
I must admit it does make me well up if I'm reading it when I feel a little tender. However I also cried at Owl Babies (when the Owl Mother Came Back) so I am not a good example at all.