al-Muhalla by Imam ibn Hazm

Al-Muhalla is one of the largest and most impressive books of Fiqh in existence. Adil Salahi from Muslim Heritage describes it thus:

This book is a wealth of scholarship, in which Ibn Hazm discusses each question separately. On each question, he cites the views of earlier scholars of high achievement, not restricting himself to the views of the four schools of Fiqh, but also citing the rulings of scholars like al-Hassan al-Basri (d. 110), al-Laith ibn Saad (d. 175), Ata' (d. 114), Sufyan al-Thawri (d. 161), al-Awza'ie (d. 157), etc. He also quotes the evidence they cite in support of their views. He then discusses why he considers their views incorrect, and produces the evidence in support of his own view. This makes for a highly scholarly discussion. Many scholars describe Al-Muhalla as the encyclopaedia of Islamic Fiqh. Indeed, it has preserved many of the views of early scholars whose work was either not documented or lost. The only problem with Al-Muhalla is that Ibn Hazm is often scathing in his criticism of his opponents. Yet there is no doubt that he is an honest defender of what he considers to be the truth. Any scholar who wants to exercise ijtihad, in order to arrive at rulings for questions encountered in present day life cannot overlook Al-Muhalla. 
However, this book is a must-Islamic Fiqh, for a public knowledgeable or advanced.

Imam Ibn Abd al-Salam said: "I did not see, in all the books of knowledge in Islam, anything like Ibn Hazm's al-Muhalla nor like Ibn Qudama' al-Mughni."

It is truly one of the most valuable Fiqh encyclopedias available, especially for advanced students of Fiqh.