The book "Orthogonal Functions" provides a comprehensive introduction to the concept of orthogonal functions in mathematics. It explains how two functions are considered orthogonal when their inner product is equal to zero. The text discusses the definition of the inner product and illustrates how this concept can be understood through the approximation of vectors. The authors, Lambert M. Surhone, Miriam T. Timpledon, and Susan F. Marseken, use high-quality content to make complex mathematical concepts accessible. This textbook is aimed at students and professionals seeking a deeper understanding of mathematical fundamentals and their applications in the natural sciences. It serves as a valuable reference for anyone interested in the theory and practice of orthogonal functions.