But to get an understanding of his approach I think a little background information is also important about the Musar Movement of Reb Israel Salanter. So for background information I think one should also learn the books of his major disciples like Isaac Blazer and Simcha Zizel from Kelm.
Then there are the larger philosophical issues about this in which it is necessary to have some background in philosophy also. The Minimum requirement for that is to have some basic understanding of the three major periods of Human philosophy-ancient Greece, the Middles Ages and German Idealism.
The idea here is that anything dealing with ethical issues in Jewish framework implies a larger context of Jewish Thought.
So before you can work on fixing faulty character traits in yourself which is the main focus of Musar you need to have a wider picture of the justification for the whole process. I mean if there is no justification for ethics in the first place then why bother with correcting your character? Just do what you want! So the issue of ethical theory does come up even in a limited subject like trust in God.
We fist need to know why are moral rules needed? For example, why do humans need rules about keeping promises, telling the truth and private property? This answer should be fairly obvious. Without such rules people would not be able to live amongst other humans. People could not make plans, could not leave their belongings behind them wherever they went. We would not know who to trust and what to expect from others. Civilized, social life would not be possible.
I in fact have found that among people that have no moral rules, it is impossible to remain.
Musar can't stand on its own. It is a first order theory. It needs a second order theory to support it.
Now normally speaking this would be simple to provide if all we needed was the Guide for the Perplexed of The Rambam and the books of Saadia Geon.
Now these are good books. But there is Kant. Without Kant we can't know the limits of what we can know in metaphysical subjects.In other words German Idealism provides a critique on the whole Musar project that needs to be answered and not ignored.