"War Horse": This is a classic for a reason. It explores the devastating impact of war through the eyes of a horse, and its themes of loss, friendship, and resilience can be deeply affecting.
"The Butterfly Lion": This story of a young boy's friendship with a lion in wartime Africa is filled with bittersweet moments and evokes strong emotions about loss and the importance of hope.
"Private Peaceful": This powerful story about a young soldier in World War I is unflinching in its depiction of the horrors of war and the loss of innocence.
"The Amazing Story of Adolphus Tips": While this story has a more whimsical tone, it explores themes of grief, loss, and the search for meaning, which can resonate with readers on a deeply emotional level.
Ultimately, the "saddest" book for you will depend on your personal preferences and what you find most moving.
It's worth noting that many of Morpurgo's books, while addressing difficult themes, also offer messages of hope, resilience, and the enduring power of love and friendship.