I recently bought A Letter for the Ages (Iggeres HaRamban): The Ramban’s Ethical Letter. This is a short (3 pages in English) letter that Nahmanides (a.k.a. Ramban) wrote to his son. I will likely share brief excerpts from time to time and maybe a reflection.
Accustom yourself to speak gently to all people at all times. This will protect you from anger — a most serious character flaw which causes one to sin.