Here is one of the most important contemporary texts on the fundamentals of Fiqh - Usul Fiqh - written by the researcher and Dr. Abd al-Karim Zaydan.

This book exposes and compares the rules of Usul Fiqh, it is a work widely spread around the world where it is often read and taught.

Book on the principes of Fiqh written by Imam Ibn 'Ammar al-Aqfahasi

Principes laid down by Muhammad ibn Hasan al-Shaybani. He was born in Wasit in 132 AH, and grew up in Kufa. He was a pupil of Abu Hanifa. Imam Shafi’i said, “I have not seen anyone more eloquent than him. I used to think when I saw him reciting the Qur’an that it was as if the Qur’an had been revealed in his language.” He also said, “I have not seen anyone more itelligent than Muhammad ibn al-Hasan.” Al-Dhahabi said, “He narrated from Malik ibn Anas and others, and he was one of the great oceans of knowledge and fiqh, and he was strong [when he narrated] from Malik.” Muhammad said, “I stood at Malik’s door for three years and I heard [the Muwatta’] from him [with] more than seven hundred hadith.” He died in Ray in 189 AH.

This work of Ibn Hazm deals with Usul al-Fiqh very detailed and thorough, very rich evidence from the Qur'an and the Sunnah and hammering the importance of sticking on both in everything.

The book covers all chapters of Usool as evidences, judgments, the use of texts, orders, prohibitions, the general and the specific, the abrogating and the abrogated, the analogy, consensus and many other topics.

Book on al-Nas and Qiyas of Usul written by Dr. Hamd al-Sa'idi in 2 volumes