It is no surprise if somebody says Gurbani as Book. This is what one sees with physical eyes. They do not lie, this is what they see. And I personally feel that it is the biggest task for most of us too, to see Guru in The Book. Most of us say it, but fail in practice. With the blessings of the Guru Himself we are able to see Guru therein.
ਪੋਥੀ ਪਰਮੇਸਰ ਕਾ ਥਾਨੁ ॥pothhee paramaesar kaa thhaan ||
This Holy Book is the home of the Transcendent Lord God.
Guru Sahib Himself use the word POTHI.
The biggest project for Humanity to accomplish is to find God in it. It is not easy job without the blessings.