My question is, why is this Ayat referring to Quran as a book?
This needs to be understood along with other ayats.
In Surah aali Imran 3:81 =
And when Allah took the covenant
of the prophets, [saying], "Whatever I give you of al-Kitab and
wisdom and then there comes to you a messenger confirming what is with
you, you [must] believe in him and support him." [Allah] said, "Have
you acknowledged and taken upon that My commitment?" They said, "We
have acknowledged it." He said, "Then bear witness, and I am with you
among the witnesses."
In surah Younus 10:37 =
And it was not for this Quran to be produced by other than
Allah, but [it is] a confirmation of what was before it and a
detailed explanation of al-Kitab, about which there is
no doubt, from the Lord of the worlds.
And in surah ar-Ra'd 13:39 =
Allah eliminates what He wills or confirms, and with Him is the Mother
of the Book.
So al-Kitab is the mother of the book with Allah, and from it is what all the revelation was revealed to the prophets, and the Quran is what explains the revelation.