In this book by Ibn Qayyim Al-Jawziyyah deals with spiritual side of tawbah without neglecting the physical requirement and conditions. This book is the most concise references that can have a big impact on you once you read it. Topics gone into detail include symptoms of the heart's sickness and signs of its health, doing without the pleasures of this world, perseverance, complete reliance on Allah, the life of this world, to mention only a few. Excellent read