Mukhtasar Usool e Fiqh by Maulana Abdul Haq Nadvi Madni
Usool e Fiqh is knowledge of the evidence for shar‘i rulings and the ways in which these rulings are derived from the evidence, in general or in detail.
The word asl (pl. usool) refers to the origin of a thing and what it is based on. Hence the father is the origin of the child, and the river is the origin of the channel.
The benefit of usool al-fiqh is that it enables a person to attain the ability to derive shar‘i rulings from the evidence by means of a sound process.