1.1. Definition of the Books in linguistic terms
The Arabic word kutub (books) is the plural of kitab (book). It comes from the root kataba which refers to collecting things and putting them together. [1] See: Ibn Faris: Maqayis al-Lughah, 5/158; Ibn al-Athir: al-Nihayah, 4/147; al-Fayyumi: al-Misbah al-Munir, 2/524.

1.2. Definition of the Books in Islamic terminology
In Islamic terminology, the kutub are the books and scriptures containing the words of Allah, may He be exalted, which He revealed to His Messengers (peace be upon them), whether they were sent down in written form, like the Torah, or they were revealed verbally through the angel, then written down after that. [2] See: A group of scholars: Usul al-Iman fi Daw’ al-Kitab wa’l-Sunnah, p. 127. See also: Muhammad ibn Nasr al-Marwazi: Ta‘zim Qadr al-Salah, 1/393; al-Bayhaqi: al-Jami‘ li Shu‘ab al-Iman, 1/345; Hafiz al-Hakami: A‘lam al-Sunnah al-Manshurah, p. 43.

Allah, may He be exalted, says, "Righteousness is not that you turn your faces toward the east or the west, but true righteousness is in one who believes in Allah , the Last Day, the angels, the Book, and the prophets." (al-Baqarah 2:177) Ibn Kathir said in his commentary on the word the Book: ‘This is a generic term which includes the Books that were sent down from heaven to the Prophets, until they concluded with the greatest of them, namely the Qur’an, which  confirmed the Scriptures that came before it, in which all good things are mentioned. It contains guidance that leads to happiness in this world and the hereafter, and by means of it Allah abrogated all the books that came before it.’ [3] See: Tafsir Ibn Kathir, 1/486