A very throughthorough (altoughalthough a bit dense) modern treatment is given in the article of Einsiedler,Lindenstrauss Lindenstrauss,Michel Michel and Venkatesh about the ergodic theoretical proof of Duke's theorem (which is not unrelated to Gregely'sGergely's notes) - http://www.ma.huji.ac.il/~elon/Publications/Erg-Duke.pdf.