Lifetimes by Brian Mellonie
This book is more about life than it is about death. Life is what comes between birth and death. Death is presented as a natural part of the cycle, not as something to fear. There is no mention in this book of an afterlife, of God, or any "place" where things go after they die. It is simple enough for a toddler or preschooler to understand.
The Tenth Good Thing About Barney by Judith Viorst
A story about a boy whose pet cat has died. One of his friends suggests the cat is in heaven, but the father states that we don't even know if heaven exists. What is the tenth good thing about Barney? Once he is buried, he will return to the earth, helping nourish the soil for flowers to grow. This could be read aloud to a preschooler or Kindergartener, but children with reading skills will probably also like this story.
The Fall of Freddie the Leaf: A Story of Life for All Ages by Leo Buscaglia
The life cycle of a leaf is a metaphor for all life. This book truly is appropriate for all ages. Although entirely secular, there is mention of going to a better place.