The book "Variational Methods in Engineering" by R. S. Schechter provides a comprehensive review of recent developments in applying variational methods to the thermodynamics of continuous media over the past decade. It includes a wide range of examples from nonequilibrium thermodynamics, fluid dynamics, and chemistry, highlighting the power of variational methods in these fields. However, the scope is limited compared to the title suggests, with a brief and somewhat imprecise presentation of the theory and limited coverage of other engineering applications. The book is well-organized, with a clear structure that helps present the subject clearly. It covers components of hydraulic servo-control systems, their performance characteristics, and the progression from simple to complex systems. Chapter 10 discusses nonlinearities caused by shaped gain, backlash, and friction, and concludes with a discussion on instability. Chapter 11 focuses on pressure and flow-control valves that use feedback, forming servo-loops. The book also addresses hydraulic power supplies, their interaction with the control process, heat dissipation, and the importance of contamination and filtration. "Hydraulic Control Systems" by H. E. Merritt is a comprehensive treatise on hydraulic and hydroelectronic control systems, filling a gap in the literature. Despite its academic level, it is valuable for practicing engineers due to its precise design guidelines for components and systems. The author uses graphical methods imaginatively and avoids overwhelming readers with equations.The book "Variational Methods in Engineering" by R. S. Schechter provides a comprehensive review of recent developments in applying variational methods to the thermodynamics of continuous media over the past decade. It includes a wide range of examples from nonequilibrium thermodynamics, fluid dynamics, and chemistry, highlighting the power of variational methods in these fields. However, the scope is limited compared to the title suggests, with a brief and somewhat imprecise presentation of the theory and limited coverage of other engineering applications. The book is well-organized, with a clear structure that helps present the subject clearly. It covers components of hydraulic servo-control systems, their performance characteristics, and the progression from simple to complex systems. Chapter 10 discusses nonlinearities caused by shaped gain, backlash, and friction, and concludes with a discussion on instability. Chapter 11 focuses on pressure and flow-control valves that use feedback, forming servo-loops. The book also addresses hydraulic power supplies, their interaction with the control process, heat dissipation, and the importance of contamination and filtration. "Hydraulic Control Systems" by H. E. Merritt is a comprehensive treatise on hydraulic and hydroelectronic control systems, filling a gap in the literature. Despite its academic level, it is valuable for practicing engineers due to its precise design guidelines for components and systems. The author uses graphical methods imaginatively and avoids overwhelming readers with equations.