Once is a novel by first time author Morris Gleitzman. It is a single book, and its length is about 224 pages. The story follows the journey of Felix and Zelda, two Jewish children in Nazi-occupied Poland in 1941. Told from Felix's perspective, the novel follows the children's trials and tribulations as they travel across Europe, trying to make it to safety. The novel delves into a complex but vital period of history, as well as exploring themes of friendship, courage and resilience. The story is gripping, poignant, and captivating, and it is sure to be appreciated by readers of all ages.