The "Who Was?" books are a popular series of children's non-fiction biographies published by Penguin Books. They tell the stories of historical figures, innovators, legends, and trailblazers in an accessible and illustrated format. The series includes over 250 titles, covering important personalities from various fields like science, politics, arts, and sports. The first books in the series were published in 2002, including titles like "Who Was Sacagawea?", "Who Was Ben Franklin?", and "Who Was Albert Einstein?". The books typically feature around 80 illustrations and aim to engage young readers with informative and entertaining content. The series has also inspired a Netflix variety show based on the books.