Q.The individual is asking, what is the best book to be studied after an individual has completed kitaab at-tawheed?
A.The shaykh said, there is no doubt from the best books/material that an individual studies after kitaab at-tawheed is that which is authentically reported on the Prophet (Sallaahu alayhi wa sallam) pertaining to matters of worship and being sincere in the affair of worship. Also those narrations or those ahadeeth (Prophetic traditions) mentioned about dealings and good character. Individuals should also read that which will encourage an individual to perform supererogatory actions of worship after the obligatory actions of worship.
Shaykh Saalih Muhammad al-Luhaydaan (May Allaah preserve him) (Kitaab-ut-Tawheed Class Q&A Session 02.02.13 Riyadh).