About the book:

One of the important texts that every student of knowledge must study is The Book of the Excellence of Islām (Kitāb Faḍl al-Islām), authored by Imām Muḥammad bin ʿAbd al-Wahhāb (may Allāh have mercy on him).
The root origin of the word Islām derives from the triliteral root sīn–lām–mīm (س–ل–م), and most usages of this root pertain to well-being and safety. Islām means submission because the one who submits is safe from stubbornness and resistance. It is also said that Islām is entry into al-silm (peace), meaning that each party is protected from harm by the other.
Islām is the surrender to Allāh, the Exalted, through Tawḥīd (His divine right to be worshipped alone) inwardly, and submission to Him outwardly through obedience, along with disavowal from shirk (associating partners with Him) and its people. It consists of three matters: submission to Allāh internally (i.e., sincerity of the heart to Allāh alone, such that one is humbled, obedient, and devoted in monotheism); outward submission (i.e., the limbs obey and comply with His commands); and disavowal from shirk and those who commit it.