The book "Voltage Controlled Ring Oscillators and their Applications" provides a comprehensive analysis of voltage-controlled ring oscillators (VCOs) and their diverse applications in high-speed systems. It explains how VCOs play a crucial role due to their ease of integration into modern technologies. In particular, the application of ring VCOs in phase-locked loops (PLLs) is highlighted, which are responsible for maintaining coherence between the output signal and the reference signal through phase comparison. The book examines various VCO designs and their performance characteristics, with a special focus on improving frequency ranges and energy consumption. The authors, Bhawika Kinger and Pradeep Kumar Ghosh, offer valuable insights into the theoretical and practical aspects of this technology, which are of great interest to engineers and researchers.