The three basic texts that R' Yisroel Salanter wanted people to learn were Chovos HaLevavosChovos HaLevavos, Mesilas YesharimMesilas Yesharim and Cheshbon HaNefesh. (source: http://rabbisedley.blogspot.co.il/2008/12/ben-franklin-and-cheshbon-hanefesh.htmlhere)
what is the difference between Chovos Halevavos and Mesilas Yesharim
which book should a beginner focus on first and why?
what about someone more advanced?