The prophet PBUH said: "If Allah intends good for anybody then he gives him understanding (fiqh) in deen". It is not possible for the one  with understanding (fiqh) to be without usul al_fiqh or for the ususli (the one who knows usul al-fiqh)  to do without fiqh if he wants to have understanding (of the deen). (shaykh Al-Uthaymin, Kitab Al-Ilm) This book is the first of the Usul Series, which consists of various books of the Principles( Usul) of the religion.