Praise be to Allah.
The best book of all for softening the heart is the Book of Allah, hence Allah calls it maw’izah (good advice) as He says (interpretation of the meaning):
“O mankind! There has come to you a good advice from your Lord (i.e. the Qur’aan, enjoining all that is good and forbidding all that is evil), and a healing for that which is in your breasts, — a guidance and a mercy (explaining lawful and unlawful things) for the believers”
There is no book that reforms hearts and heals them of disease like the Qur’aan. Hence the Muslim should not turn away from it towards other books. The Muslim should read Qur’aan a great deal with proper focus and humility, pondering the meanings. He will see the effect of that on his heart. He can also refer to one of the brief tafseers in order to understand the meanings of any verses that he finds difficult, such as the Tafseer of al-Sa’di (may Allah have mercy on him).
With regard to what the scholars have written on al-raqaa’iq (heart softening reports), their writings are many and varied. Some form parts of other books and some form separate books. In the books of hadeeth such as Saheeh al-Bukahari and Saheeh Muslim etc there are chapters on al-raqaa’iq. With regard to separate books, we have chosen the following list for you, but it should be noted that these books are simply a help: there is nothing wrong with the Muslim reading them and benefiting from them, but that should not replace reading and studying the Qur’aan.
Al-Bahr al-Raa’iq fi al-Zuhd wa’l-Raqaa’iq by Ahmad Fareed
Al-Zuhd wa’l-Raqaa’iq by ‘Abd-Allah ibn al-Mubaarak, edited by Habeeb al-Rahmaan al-A’zami.
Al-Fawaa’id wa’l-Zuhd wa’l-Raqaa’iq wa’l-Maraathi by Ja’far ibn Muhammad ibn Nusayr al-Khaldi, edited by Majdi Fathi al-Sayyid.
Maw’izah al-Mu’mineen min Ihya’ ‘Uloom al-Deen by Muhammad Jamaal al-Deen al-Qaasimi
Tareeq al-Hijratayn wa Baab al-Sa’aadatayn
These three books were all written by Ibn al-Qayyim.
Lataa’if al-Ma’aarif by Ibn Rajab.
Muqtatafaat min al-Mawaa’iz wa’l-Adab by ‘Ali Saalim Aal Haarith.
Al-Ta’leeq ‘ala Manzoomah fi’l-Sayr ila Allah wa’l-Daar il-Aakhirah by Shaykh ‘Abd al-Rahmaan al-Sa’di.
Tazkiyat al-Nafs by Shaykh al-Islam Ibn Taymiyah, edited by Dr. Muhammad Sa’eed al-Qahtaani.
Al-Sirr al-Maknoon fi Riqqah al-Quloob wa Dam’ al-‘Ayoon by ‘Abd al-Kareem al-Deewaan, Dar al-Muslim.
Mawaarid al-Zam’aan li Duroos al-Zamaan by ‘Abd al-‘Azeez al-Salmaan.
Al-Tadhkirah fi Ahwaal al-Mawta wa Umoor al-Aakhirah by Abu ‘Abd-Allah al-Qurtubi al-Mufassir, edited by Mahmoud al-Bastawaysi
Ahwaal al-Qiyaamah by ‘Abd al-Malik Kulayb
Al-Qabr wa ‘Adhaabuhu wa Na’eemuhu by Husayn al-‘Awayishah [available in English under the title The Grave: Punishment and Blessings]
And Allah is the Source of strength.