For my understanding of modular forms, I have "properly" read the last chapter of Serre's book on arithmetic. I have not "properly" read the setup of Hecke operators for congruence subgroups.
I was intending to read the paper of Atkin and Lehner for filling this lacuna, as well as for understanding the proof of Atkin-Lehnher theorem. For some reason I could not relate very well with the exposition in the book of Diamond & Shurman or the book of Shimura.
I would be most grateful if somebody can point out some good reference for the Atkin-Lehner theorem other than these.