Even though the Musar movement was based on the idea of learning the four basic Musar books חובות לבבות, אורחות צדיקים, שערי תשובה, מסילת ישרים ,  I found the Light Of Israel and other books by the disiples of Rav Israel Salanter to be of the most help to me. Musar also helps to understand what Torah is all about. It is all too easy to go off on confused  directions because of lack of understanding the hierarchy of principle in Torah.  

[Though I do find the books of Rav Nahman very helpful even though they are  not a part of the regular Musar books still I should add that I think Rav Nahman is mostly good if people already have a good background in Gemara, the Ari, and Musar. --But I do not deny that he can be helpful for everyone also. Still without the prerequisites, there is a tendency to go off in odd directions.

[Musar is based on the idea of learning these books of morals from the Middle Ages, Obligations of the Hearts, Gates of Repentance, Ways of the Righteous and also the book of Rav Moshe Haim Lutzato The Ways of the Just.