The Immortal Jellyfish by Sang Miao.
There are a few books from Goodbye Mog to Stardust that try to broach the subject of loss and death and this book is another one.
A little boy has a fantastic relationship with his grandfather who early on in the story sadly passes away. Grandfather appears to the boy in a dream however and shows how people and animals are reborn in whatever form they choose to be.
It’s as nice a way as possible to talk about mortality and manages to be rather inspirational too.
The illustrations are very atmospheric and perfectly reflect the abstract realm of much of the book.
Touching and thought provoking.