Automorphic Forms on GL(2)

Automorphic Forms on GL(2) is a book by H. Jacquet and Robert Langlands (1970) where they rewrite Hecke's theory of modular forms in terms of the representation theory of GL(2) over local fields and adele rings of global fields. A second volume by Jacquet (1972) gives an interpretation of some results by Rankin and Selberg in terms of the representation theory of GL(2) × GL(2).