This book explains one of the most comprehensive hadeeths of the Prophet (peace be upon him), the Hadeeth of Angel Gabriel. It touches upon almost every deed of Islam. Qaadhi Iyadh has pointed out that this hadith covers all of the aspects of internal and outward acts of worship. It touches upon the deeds that are related to the external organs as well as that of the heart. Indeed, he stated, "It covers the religion to such an extent that all the religious sciences are found in it and branch out from it."
The hadith is also known to be called Umm al-Sunnah (the foundation of the Sunnah) in the same way that Surah al-Faatiha is called Umm al-Kitaab (foundation of the Book). In the same way that Surah al-Faatiha encompasses the meaing of the Quran as a whole, this hadith encompasses the meaning of the sunnah as a whole.
The commentary of this hadith, extracted from the author's Commentary on the Forty Hadith of al-Nawawi, is one of many such commentaries throughout the history of Islam. It is, though, the first such commentary available in the English language.