Always and Forever
Always remember the good times.
It's a sad book, but anything about death and mourning really has to be. Four animals live in a house together. One of the animals, Fox, dies, and the other family members are in such grief that they find it hard to go on with their lives. One of their friends comes by and visits and kind of spurs them into remembering funny things, and the good things about Fox, and they start remembering what it's like to live again. They decide to do things in Fox's memory and that helps them feel connected to him, and to continue their lives. And that's how it ends.
It's a really good depiction of what it's like to lose someone. There's nothing religious to it, nothing existential, just a very relatable depiction of the pain you feel when someone dies, and how you have to move on, which is not disrespectful of the one who died.