The present volume of The Pocket Mathematical Library continues the study of infinite series begun in its companion volume Infinite Series: Rudiments, by the same author. Together the two volumes give a detailed treatment of the theory of nu­ merical series, i.e., infinite series whose terms are numbers. The picture is then completed by a third volume, entitled Functional Series, which, as its name implies, is devoted to the study of infinite series whose terms are functions. The set of three vol­ umes makes up a comprehensive treatise on all aspects of a key topic of pure and applied mathematics.As in the companion volume, the problems appearing at the end of each section constitute an important part of the course, and should not be neglected by the serious student.