The book "Order of Chemical Clusters: Skeletal Numbers and Cluster Trees" offers a comprehensive analysis of the categorization of chemical structures through the methods of 4N and skeletal numbers. It explores the connection between these categorization approaches and the geometric structures of clusters, particularly their symmetry. The skeletal numbers, derived from the 4N series, allow for a simplified representation of complex molecules and large clusters through a single number. The application of these methods is illustrated with examples such as palladium carbonyl clusters and various boranes. The book also includes a K(n)-cluster map that highlights the prediction of cluster symmetries through skeletal numbers. This innovative approach provides valuable insights into the structure and properties of chemical clusters for both students and professionals in the field of chemistry.