The 2nd verse of Baqarah say: "this is the book", But wasn't the Quran sent down as a "revelation"?

Al-Baqarah 2:2 states: >This is the Book; in it is guidance sure, without doubt, to those who fear Allah; Keeping in view of the above Ayat, wasn't the Quran sent down as a "revelation" to Prophet Muhammad? Furthermore, it wasn't compiled as a book till several years till after his death. My question is, why is this Ayat referring to Quran as a book?