There are obvious choices for this one, like Marley and Me. But only the heartless ones don't cry when that dog dies.
Anyway, this book makes me cry just because you know he's going to die and you just have to sit around and wait for it. And I can only imagine how Henry would feel about it, knowing his days were numbered.
What really makes me cry about it is that she gets to see Henry one last time when she's eighty or something like that. (In the movie, the meadow scene at the end makes me bawl like a toddler.) It's just so heartwrenching. And I'm a hopeless sap.