order book vs order of the day

order book and order of the day both are nouns.

Word	Noun	Adjective	Verb	Adverb
order book	Yes	No	No	No
order of the day	Yes	No	No	No

As nouns, order of the day is a hypernym of order book; that is, order of the day is a word with a broader meaning than order book:

order book (noun)	order of the day (noun)
a book in which customers' orders are entered; usually makes multiple copies of the order	the order of business for an assembly on a given day
a printed copy of the order of the day