Shaari Teshuva by Rabainu Yona

Ethics. {Musar}. I think  one of the most important Musar books to read is the Shaari Teshuva [Gates of Repentance] by Rabainu Yona. It occurred to me that it contains all the basics of Torah in a small, thin volume. Just think about it. It goes through the stringency of לאווין [prohibitions] that is a very important aspect of Torah. Then it goes through pretty much all the aspects of Torah that are relevant today that most people do not know about. And then it gets into the ארבע כיתות that deals with the four types of people that will never see the Divine Countenance. Those that speak lies, lashon hara, ליצנות, flattery.

If you think about it you can see it contains most of the basic facts that one needs to know in order to understand and keep the Torah. That is pretty remarkable for one small book to do.