it is said that it means Al-Lawh Al-Mahfuz, (the Preserved Table).
It is also said that is refers to the divinely revealed inscribed Books that people recite, and this is why Allah said,
فِي رَقٍّ مَّنشُورٍ
28 English Literal
By/and a Book written/inscribed/drafted
29 Almuntakhab Fi Tafsir Alquran Alkarim
And by the Sacred Book inscribed by the scribes of Allah who put into words all that He pronounces to them and accords with deep respect, reverence and veneration