Serge Lang 
Introduction to Modular Forms [PDF ebook] 

From the reviews: "This book gives a thorough introduction to several theories that are fundamental to research on modular forms. Most of the material, despite its importance, had previously been unavailable in textbook form. Complete and readable proofs are given… In conclusion, this book is a welcome addition to the literature for the growing number of students and mathematicians in other fields who want to understand the recent developments in the theory of modular forms." #Mathematical Reviews#"This book will certainly be indispensable to all those wishing to get an up-to-date initiation to the theory of modular forms."#Publicationes Mathematicae#