This book is extremely important in explaining the true methodology of adherence to Allah's (swt) Book, the Sunnah of His Messenger, and the Guidance of the sahaabah.
Ibn ul-Qayyim starts by explaining the qualities of righteousness and piety that are required from everyone, both in practice and in dawah. He then goes on to describe the true meaning of migration to Allaah(swt) and His Messenger, discussing in the process full submission to the Messenger's Sunnah as an integral requirement of eemaan. He also talks about the situations of true misery and true happiness. He also specifies the requirement of the Journey of Migration, along with some important insights for sound comprehension of Quran. This book is very touching in its address to the soul and heart.
Other Sources:
Who Wrote the Quran? Proving that the Quran is the word of God (Allah)
Why a Dua doesn't get answered?