I am looking for a highly recommended reference for the Theory of Modular Forms for Normal Subgroups of Finite Index of $PSL(2, \mathbb{Z})$.
Many references, that have been mentioned before on this website, discuss the Theory of Modular Forms for $SL(2, \mathbb{Z})$ and Congruence Subgroup of $SL(2, \mathbb{Z})$ (I don't need them at all).
All I need is a reference to understand this part of the article, starting from "1.2. Modular Forms...".
Where $G=PSL(2,\mathbb{R})$.
