(or: How Sassy Peach's advance review of HAVE BOOK-WILL TRAVEL proves my point.)
I often wonder whether it is at all possible to create a book that readers will enjoy reading (enjoy, like in "really having fun"), unless you had a great time writing it. This is not something that worried many classic writers; Dostoyevsky, to mention just one, was probably never happier than when writing about someone who was going to hang himself in the barn - but he could't have had much fun planning the setup.
Now, I always enjoy writing (even when the subject is serious), but working as a team with my son was simply exhilarating. I can't remember ever laughing more than when we were thinking up the next chapter together.