This book provides a comprehensive overview and deep technical insights into 6G NTNs, with a particular emphasis on network architecture, resource management, and performance evaluation. Topics covered include hierarchical MEO-LEO mobility management, UAV-assisted LEO backbone construction for data collection, cybertwin-assisted access mode selection, service function chain-based quality of service (QoS) guaranteed resource orchestration, deep reinforcement-based RAN slicing, satellite-aircraft handover, and high-capacity multicast beamforming. Additionally, the book introduces simulation platform named UltraStar and QUIC-based streaming frameworks to facilitate performance testing and design validation. The content is systematically organized into network architecture, physical-layer optimization, access management, routing and transmission, resource management, and simulation, aiming to provide readers with a holistic perspective on the emerging field of ultra-dense LEO-based NTNs for 6G. While primarily targeted at academic researchers and industry practitioners in satellite and wireless communication, the book is also suitable for graduate students seeking to explore the forefront of 6G network innovation.