It deals with vital and urgent issues that are modern.
I mean to say the basic set of ethics books from the middle ages are very good, but they tend not to deal with some issues that came up after that period.
Some of the issues are Bitul Torah. That is not just the mitzvah of learning Torahת but the sin of not learning Torah when one can be learning. A detailed discussion of what is idolatry. It also shows the way of prayer and service in the way of the Arizal (Isaac Luria).
I think it is, possibly, the most important Musar book ever to be written, because of its dealing with the issues that are urgent, but just were not relevant during the Middle Ages.
The actual books of Musar that are best are the known set of Mediaeval ones, plus the disciples of Reb Israel Salanter who wrote more modern approaches like the Madgragat HaAdam [מדרגת האדם] about trust in God and Isaac Blasser about fear of God. That was the general tendency of the Musar movement for each person to find some trait the felt they needed to work on and to hold onto, and often they would write a lot about that particular issue. I myself did not really go for any of that and went off in different directions, but I did find one trait that I have tried to hold onto at all cost--to speak the truth always no matter what. This actually had an early beginning when I read the Musar book אורחות צדיקים The Ways Of The Righteous.
[There has been good stuff in terms of Musar. Recently someone found more writings of Isaac Blazzer, the major disciple of Reb Israel Salanter, and printed them. It is almost impossible to find, but I saw a copy in Netivot. [So from him we have two books- אור ישראל and this newer book.] There is a new and well researched biography on the Gra also that came out about 7 years ago- which I have heard is very good. It must be good judging by how many people were enraged at it.
Everyone in some official position condemned the authors even though they had done meticulous research.You can see it must be the best book on the subject of the Gra.
I did not see the book myself but from what I understood the conclusion of the authors was that the Gra meant what he said about the excommunication. This apparently got everyone in Israel very angry at the authors, and now you can't find this book anywhere. [There were previous books about this issue. But this was a modern book that took the stance that the Gra meant what he said. and that the cherem has halachic validity.