In the holy Quran, Quran is mentioned numerous time as the book
It is He who has sent down to you, [O Muhammad], the Book; (3:7)
In the last sermon of the prophet, the prophet said
I leave behind me two things, the QUR'AN and my example, the SUNNAH and if you follow these you will never go astray.
But Quran was never compiled as book in the life of Muhammad (PBUH). Why was it left out of the book form when the Quran itself refer to it as a book?