What is This?
This is a book by Ken Ribet and William Stein entitled "Lectures on Modular Forms and Hecke Operators".
This book began when the second author typed notes for the first author's 1996 Berkeley course on modular forms with a view toward explaining some of the key ideas in Wiles's celebrated proof of Fermat's Last Theorem. The second author then expanded and rewrote the notes while teaching a course at Harvard in 2003 on modular abelian varieties.
The intended audience of this book is advanced graduate students and mathematical researchers. This book is more advanced than those of Lozano, Stein's Intro to Modular Forms, and Diamond-Shurman, and at a relatively similar level to Diamond-Im, though with more details. It should be substantially more accessible than a typical research paper in the area.