“And the most important book after the Book of Allah – the Exalted – is the ‘Sahih’ of al-Imam al-Bukhari (may Allah have Mercy on him and be Pleased with him). As for what comes after this, the most important books regarding the Islamic sciences in my opinion are four, and they are:
1 – ‘Tafsir al-Qur’an al-‘Adhim’ by al-Hafidh Ibn Kathir (died 774 H):
…and it is the most important book of Tafsir by way of the narrations of the Salaf, since he has followed the proper methodology in explaining the Qur’an, and that is to explain the Qur’an by the Qur’an, then by the Sunnah, then by the statements of the Companions and Tabi’in, as he pointed out in his introduction, which was excerpted from Ibn Taymiyyah’s ‘Muqaddimah fi Usul at-Tafsir.’
And Ibn Kathir has based his ‘Tafsir’ upon that of Ibn Jarir at-Tabari, which the scholars have unanimously agreed is the loftiest of all tafasir, by far. However, Ibn Kathir left out many of the chains of narration mentioned by at-Tabari, and added to that some elements of the ‘Tafsir’ of Ibn Abi Hatim ar-Razi (the author of ‘al-Jarh wat-Ta’dil’), as well as that of the noble Imam Ibn Mardawiyyah. So, his ‘Tafsir’ is very comprehensive, in addition to his aiding of the belief of the Salaf, and his inclusion of excellent Fiqhi discussions.
And Ibn Kathir is a Salafi scholar of Hadith and Fiqh, whose correctness is much, and whose mistakes are few, and his ‘Tafsir’ is from the greatest of books that assist one in understanding the Book of Allah – the Exalted – and it is printed in four volumes.
2 – ‘Fath al-Bari fi Sharh Sahih al-Bukhari’ by al-Hafidh Ibn Hajar al-‘Asqalani (died 852 H):
…and it is the most important book explaining Hadith, by far, considering that he has collected, in every chapter – in addition to what al-Bukhari narrated – the most important ahadith related to each chapter that are found in the other books of the Sunnah, from the other six books, the Masanid, etc., just as he mentioned the most notable of what the previous explainers of ‘Sahih al-Bukhari’ had said. He also quotes from the explanations of the other six books, such as an-Nawawi’s ‘Sharh Sahih Muslim,’ al-Khattabi’s explanation of ‘Sunan Abi Dawud,’ and al-Baghawi’s commentary in ‘Sharh as-Sunnah,’ etc. This is all in addition to his detailed analysis of various Fiqhi issues, in which he mentions the opinions of the various madhahib on them. He also has vast, detailed linguistic discussions, as well as discussions on issues related to Hadith terminology and the principles of Fiqh.
His followers criticize him for leaning towards the madhhab of the Ash’aris in regards to issues of belief, as well as those related to the Attributes of Allah, and this is very evident in his explanations of the books of ‘Iman’ and ‘Tawhid’ of ‘Sahih al-Bukhari.’ However, this can be remedied by simply familiarizing one’s self with the madhhab of the Salaf in regards to these issues. Likewise, his followers criticize him for his sometimes blatant partisanship to his Shafi’i madhhab in rulings that sometimes contradict clear proof. For example, look to his explanation of the chapter on wiping the head, in the book on ablution (hadith # 185).
‘Fath al-Bari’ is printed in fourteen volumes, including its introduction, ‘Hadi as-Sari.’
3 – ‘al-Mughni’ by Ibn Qudamah al-Maqdisi al-Hambali (died 620 H):
…and it is an encyclopedia of Fiqh that mentions the madhahib of the scholars – from the Companions, Tabi’in, and those who came after them of the four madhahib – and this is in regards to every single issue of Fiqh. He then points out the strongest of these opinions in regards to each issue, based on the proofs. He often lends strength to his own Hambali madhhab, and he sometimes opposes it, and not everything he considers to be the strongest opinion is as such, and he often supports his choices in regards to the strongest opinions with those of Ibn Taymiyyah, as we will show, in Sha’ Allah.
‘al-Mughni’ is printed in a single volume, and is also available with its explanation, ‘ash-Sharh al-Kabir,’ in twelve volumes. It is also accompanied by its index, which is printed in two volumes, and this greatly assists in searching through the book.
4 – ‘Majmu’ al-Fatawa’ of Shaykh al-Islam Ibn Taymiyyah (died 728 H):
…and it was collected and organized by ‘Abd ar-Rahman bin Muhammad bin Qasim an-Najdi al-Hambali, and it is printed in thirty-five volumes, in addition to the index, which is two volumes.
The significance of this book is based on three things:
First: its comprehensiveness and detail in regards to the madhhab of the Salaf regarding the various issues of belief, in addition to the fact that he mentions and refutes the statements of the innovated sects. It is mentioned in the fourth chapter that Ibn Taymiyyah had collected everything written by the Salaf regarding matters of belief, and that all those who wrote after him regarding the belief of the Salaf used Ibn Taymiyyah’s writings as their source.
Second: his frequent mention of the Shar’i evidences for his statements and quotes, to the point that this became his consistent methodology. And I had mentioned beforehand that I strongly advise the reading of the works of Ibn Taymiyyah and Ibn al-Qayyim, due to what that would entail of training oneself to take statements based on their evidences, and having this become an ingrained methodology for the student of knowledge.
Third: his comprehensiveness in indicating the strongest opinions in the majority of the Fiqh issues he discusses, and most of his decisions are correct, except for a very small number of them.
So, in my opinion, these are the four most important books of Islamic knowledge, and Allah Knows best. I consider them to be of this prestige due to their covering all of the Islamic sciences – original, and derived – as they contain Tafsir, explanation of Hadith, Fiqh, belief, heart-softeners, manners, adhkar, biographies, stories of battles, virtues of personalities, etc. So, the one who can absorb what is in these books has not missed anything noteworthy in regards to Islamic knowledge, since whoever wishes to look up a Shar’i matter and collects the relevant information that is in these books has not missed any bit of knowledge in regards to the given matter, generally speaking.”
[‘al-Jami’ fi Talab al-‘Ilm ash-Sharif’; 2/486-487]