Adab al-Nufus by Harith ibn Asad al-Muhasibi

Severing the love of this world from his heart. If he does that, it will be easy for him to abandon this world and easy for him to seek the afterlife... Adhering to humility, filling the heart with piety, sustaining sorrow, and being much concerned with what is coming to him. This book, despite its small size and simple language, is one of the best introductions to Sufism.