“Throughout the text there are numerous exercises, often with complete solutions or helpful hints … . The brief notes at the end of each chapter provide useful additional comments and references to the literature. Overall, this is an interesting and engagingly written text on spectral theory and its applications. Readers with the requisite background knowledge will find in this book a wealth of interesting applications of classical spectral theory, often to problems of current interest in research.” (David Seifert, zbMATH 1505.47001, 2023)