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!!
This book helped me so much. I read it in Ramadan and it opened my eyes to so many things. The light this book holds will make you look within yourself and cry. You'll want to drop your sins and reading it in solitude is the most blessed thing that can happen to you.
Tawbah gives us Quran verses, Hadiths, Stories of the Sahaba, and a lot of analogies on seeking forgiveness, finding Allah and the importance of quick repentance.