Rav Kook [whose yahrtzeit is tonight] said that he worked VERY HARD on Oros Hatshuva and remarked that if one learns it properly, then he will find light on every word. On Motzei Yom Kippur 1934 the Rav said that "One must learn Oros Hatshuva without limits".
Among other ideas in the sefer, the Rav teaches how teshuva must be with simcha and that it is not about running AWAY from life but rather RETURNING to life.
[The Holy Pnei Menachem of Ger used to learn Oros Hatshuva and someone told me that he saw other Sifrei HaRav ztz"l of the Pnei Menachem that were filled with comments in the margins.]
