Modular Functions and Dirichlet Series in Number Theory (Graduate Texts in Mathematics) (v. 41)
From the reviews of the second edition:“Apostol is an excellent writer of mathematics and the topics that are covered in this book are covered thoroughly in a concise, precise manner. … the writing is characterized by its easy, readable, fluid style. Each chapter is complemented with a nice set of exercises.” (Álvaro Lozano-Robledo, The Mathematical Association of America, June, 2011)