Why is it said, "a book sent down by God," with the possibility that revelation was a word transmitted by an angel or direct communication without a written book being sent down?

There is no impediment to naming the speech revealed by the angel a "Book," for the term "Book" encompasses both written and spoken collected speech. The word "Book" is applied to that which is inscribed in the Preserved Tablet, just as it is applied to the revelation sent down to the prophets, whether it was heard or written, such as the tablets of the Torah and the Holy Qur’an, which Gabriel brought down to the heart of the Prophet, may Allah’s prayers and peace be upon him.