Invariant Subspaces of Linear Transformations in Hilbert Space by Louis de Branges delves into the study of invariant subspaces for linear transformations in Hilbert space focusing on Volterra transformations and completely continuous operators. The book explores the relationship between invariant subspaces and integral representations of transformations providing insights into their existence and properties. It discusses the results of Russian mathematicians in this field and presents the author's unpublished work. Additionally it addresses factorization problems for operator-valued analytic functions and their connection to invariant subspaces emphasizing the importance of complete continuity in function theory.