Disturber of the Hearts by Ibn al-Jawzi
The perceptive scholar is skillful at diagnosing a range of spiritual illnesses from sins, and treating the malaise in the heart.
If a person is failing into despair, the author tries to ignite the spark of hope in the heart. On the other hand, if he sees that one is in a reckless and fearless state, he works to in still the fear of Allah in his heart.
A very interesting book for one to cure the sickness of their heart.