"Functional Calculus" is a comprehensive book that offers readers a thorough exploration of the subject, covering fundamental concepts, techniques, and applications. By establishing a solid foundation in calculus and functional analysis, the book ensures a clear understanding of the underlying principles. With a focus on topics such as operator theory, spectral theory, and Banach spaces, readers are guided through a wide range of essential areas. Through rigorous explanations and numerous examples, the book enables readers to develop a deep comprehension of functional calculus and its practical significance in solving problems involving operators, transformations, and function spaces. This invaluable resource is tailored to students, researchers, and professionals seeking to master functional calculus and apply its principles across various disciplines in mathematics and beyond.