“The exposition of this book is clear, structured and almost self-contained. Several results related to Fredholm and spectral theories for Hilbert space operators are discussed in detail. A rich bibliography is provided at the end of each chapter. The book is addressed to students, researchers in the field of spectral theory of linear non-self-adjoint operators, and mathematicians interested in applications in mathematical physics. I think that pure analysts will have some new problems to tackle.” (Bilel Krichen, Mathematical Reviews, September, 2022)<br />“This book provides a very good collection of results for the study of the structural property of unbounded linear operators with compact resolvent, in particular, for the study of non-selfadjoint operators in Hilbert spaces.” (Gen Qi Xu, zbMATH 1483.47001, 2022)