This book provides a comprehensive overview of the emerging technology of reconfigurable intelligent surfaces for wireless communications, detailing the latest advances in their hardware architectures and modes of operations, ranging from metasurface-based smart reflectors and extremely large multi-antenna transceivers, as well as multi-functional metasurfaces, along with their realistic modeling and the state-of-the-art advanced signal processing and machine learning approaches for their operation optimization. It also covers the rapidly evolving topics of beyond diagonal and transmissive reconfigurable intelligent surfaces, stacked intelligent metasurfaces, as well as dynamic metasurface antennas and holographic MIMO that have lately produced numerous research activities and relevant literature. The book also covers various applications of reconfigurable intelligent surfaces, including multi-user and near-field communications, electromagnetic imaging and sensing, over-the-air computation, physical-layer security, simultaneous wireless information and power transfer, as well as integrated sensing and communications. The current status and future directions on the book's core technology in relevant activities in standardization are also discussed.