But I also realize there are more modern issues that came up after the original set of Musar books was written. Thus I found the Nefesh Hachaim from a disciple of the Gra to be very helpful.
I did notice in Israel that some people still take the idea of the Musar Movement seriously. For example in Netivot, I saw the yeshiva of Rav Montag had an actual "Musar room" --the first time I had ever seen such a thing.
I also heard from Shimon Buso [a grandson of Bava Sali] and the daughter of Bava Sali, Avigail Buso a lot about the importance of Musar. But that was really just confirming what my impression already was about the importance of Musar. [The daughter of Bava Sali also mentioned to me about the importance of the books of Joseph Karo, i.e the Tur with the Beit Joseph etc. ]
There is also a promise of healing [physical and spiritual] that Isaac Blazzer said in the beginning of his book אור ישראל which he brings from the Rambam.
I imagine some people feel they are born perfected, and thus do not need to hear rebuke. But for the rest of us mere mortals, this seems to me to be the best way to go about character correction.
Why you might ask is all this necessary? Why not just open up the Old Testament and see what it says? The reason is that any text without background is infinitely under-determined. It can mean anything you want. See John Searle in his theory of the Background and Kelley Ross's critique in which is shows the importance of John Searle's idea.
[This is relevant for anyone who cares about the Torah. The truth is without the background, it is radically undetermined. The nice thing about the Middle Ages was that understanding in a complete and logically rigorous way the entire written and oral law was of the greatest importance for people. So the books of Musar then were written with the whole picture in mind. ]