Four brave siblings were searching for a home – and found a life of adventure! Join the Boxcar Children as they investigate the mystery of a secret window in this illustrated chapter book series beloved by generations of readers. A family moves into the house next door! The Boxcar Children decide to make friends with their new neighbors by building a tree house with them. In the process, they notice a window in their neighbor's house that nobody knows about. Does the old home have a secret? What started as a single story about the Alden Children has delighted readers for generations and sold more than 80 million books worldwide. Featuring timeless adventures, mystery, and suspense, The Boxcar Children® series continues to inspire children to learn, question, imagine, and grow.