Dar Ibn Hazm have recently published an excellent edition of Ibn Hazm’s al-Muhalla in 19 volumes. Whilst it’s still early days, it looks like a contender for being the critical edition. Scholars and students alike are currently reviewing it and a proper assessment might take a year or so.
Anyhow, it reminds me of a famous quote by Ibn ‘Abd al-Salam, recorded by al-Dhahabi in his Siyar:
‘I have not seen, in all the books of knowledge in Islam, anything comparable to Ibn Hazm’s (1) al-Muhalla nor Shaykh Muwaffaq al-Din (Ibn Qudamah)’s (2) al-Mughni.’
‘Shaykh ‘Izz al-Din spoke the truth! And the third is al-Bayhaqi’s (3) al-Sunan al-Kabir, and the fourth is Ibn ‘Abd al-Barr’s (4) al-Tamhid. Whoever acquires these (four) volumes, whilst being an intelligent mufti, and persists in reviewing them, then he truly is a scholar.’ (Siyar 18:193)
What they all have in common is an encyclopedic approach to hadith and fiqh, truly striving to arrive at the essence of the Sunnah. Now then, a number of later scholars, after quoting the above, have attempted to extend the list to include other candidates for absolutely essential masterpieces that a scholar cannot do without.
The most obvious candidate for the fifth also happens to be the most commonly cited, namely Fath al-Bari by Ibn Hajar al-Asqalani.
Muhammad al-Shawkani, who was considered to be a mujtahid (independent jurist) was asked why he hadn’t written a commentary on Sahih al-Bukhari, as other scholars (some of arguably lesser standing) had done, and replied by paraphrasing the hadith: ‘la hijra ba’d al-fath.’ i.e. ‘(There is) no (reward for) migration (to Madinah) after the conquest (of Makkah).’ By this he meant that after Fath al-Bari, no other commentary on Sahih al-Bukhari was necessary, as Ibn Hajar had accomplished all that could be accomplished.
Another book mentioned, though overlooked by al-Dhahabi (perhaps because it is incomplete), is al-Majmu’ Sharh al-Muhadhab by al-Nawawi. Ibn Kathir said of it in his entry on al-Nawawi in al-Bidayah wa al-Nihayah that he does not know of any book of fiqh better than it.
Also frequently mentioned are al-Awsat and its abridgement al-Ishraf by Ibn al-Mundhir, both of which were abridged from the lost al-Mabsut. Although both have been published in recent decades, both publications are incomplete (with a substantial amount of volumes from the manuscripts left unedited and unpublished). The sections that have been published, however, leave nobody in doubt about the value of the books and the indisputable mastery of ijtihad that Ibn al-Mundhir had. Ibn Hazm considerd him to be one of the few mujtahid mutlaqs to come after the time of Ahmad b. Hanbal and Ishaq b. Rahuwayh.