A book on any mathematical subject beyond the textbook level is of little value unless it contains new ideas and new perspectives. Of course it helps to include new results, provided that they give the reader new insights and are presented along with known old results in a clear exposition. It is with this philosophy that the author writes this volume. The two subjects, Dirichlet series and modular forms, are traditional subjects, but here Goro Shimura treats them in both orthodox and unorthodox ways. Regardless of the unorthodox treatment, the author has made the book accessible to those who are not familiar with such topics, by including plenty of expository material. The book contains never before published elementary proofs. It is self-contained, and suitable for use in a classroom setting.