The Satmar Rebbe zy'a said, "When people travel to their summer homes, it is hard to carry along many mussar sefarim. But they take along a Chumash Devarim, which is sufficient because Chumash Devarim is the best mussar sefer."
The Yid HaKadosh of Peshischa zt'l studied Chumash Devarim every day of the year and encouraged his chassidim to do the same. He said, "There isn't a better mussar sefer!" (Shem MiShmuel, Devarim 5675; Pri Tzaddik, Devarim אלה ה"ד.)