Tafsir of At-Tur 52:2

This means inscribed in a manner of order, for satr (a line/row) is the arrangement of written letters. The intent behind it—according to what al-Farra’ stated—is the book in which deeds are recorded, which the servant is given on the Day of Resurrection in his right hand or his left, and it is the one mentioned in the words of the Exalted: "And We will bring forth for him on the Day of Resurrection a record which he will encounter spread open" (17:13).

Al-Kalbi said: It is the Torah. It has also been said: It is the Torah, the Gospel, and the Psalms. It has also been said: It is the Quran. And it has been said: It is the Preserved Tablet (al-Lawḥ al-Maḥfūẓ).

In al-Baḥr, it is stated that one should not insist upon any of these specific interpretations; rather, they are presented as possibilities. The use of the indefinite form (al-tankīr)—it is said—denotes the singularity of the type, provided one holds the view of its multiplicity; or it denotes the singularity of the individual, provided one holds the opposing view. Its benefit is to indicate its specification from the genus of books by a matter that distinguishes it from all others.

The most appropriate [approach], regarding both aspects of the indefinite form if it is taken to refer to one of the two books—namely the Quran or the Torah—is that it is of the category of "[a] people" (as in: "That He may recompense a people" [45:14]). Thus, in the indefiniteness lies a perfection of definiteness, alerting one that such a book remains known whether it is expressed indefinitely or definitively. Of this same category is the indefiniteness in the words of the Exalted: [a Messenger/a Book].