Dear readers, peace
I respectfully disagree with all the explanations put forth by the esteemed forum members to abdullah's point.
I will try to explain my understanding, and the god knows best.
THE BOOK in the quran can have different meanings depending on the context. It can be referring to the quran, or the master book containing record of everything, or the book containing a record of our deeds which is part of the master book.
What does "The Book" in 40:70 refer to?
Those who deny the Book (al-kitaab), and what We have sent Our messengers (plural) with. Therefore, they will come to know. (40:70)
The context is clearly the Last Day (40:71-77)
"The Book" in 40:70 cannot be the quran because of the contradiction it causes as per abdullah.
"what we have sent our messengers with" in the same sign cannot be the signs in the horizons and within ourselves because those signs were not sent with the messengers
"what we have sent our messengers with" cannot be only the quran because of the word "messengers" plural.
"what we have sent our messengers with" cannot be the hadeeth of the messengers because then we will need the authentic hadeeth of ALL messengers for our salvation not just of Muhammad.
Careful consideration of the quran reveals that "The Book" in 40:70 can only refer to THE BOOK in which our deeds are being recorded which is a part of the master record in which everything is recorded. This understanding is solidified by the context of 40:70 which is the Last Day.
Please ponder on the following signs:
Who is more wicked than he who invents lies about God, or denies His revelations? These will receive their recompense from The Book (al-kitaab); so that when Our messengers come to terminate their lives, they will say: Where are those whom you used to call on besides God? They said: They have abandoned us! And they bore witness upon themselves that they were rejecters. (7:37)
Context is the Last Day just like 40:70
And if We let the people taste a mercy after some harm had afflicted them, they take to scheming against Our revelations! Say: God is faster in scheming; Our messengers record (yaktaboon) what you scheme. (10:21)
They record where? In THE BOOK obviously
And The Book (al-kitaab) was displayed, so you see the criminals fearful of what is in it. And they say: Woe to us! What is with this record that it does not leave out anything small or large but has counted it. And they found what they had done present. And your Lord does not wrong anyone. (18:49)
Same context of The Last Day
No, We will record (sanuktab) what he says, and We will increase for him the retribution significantly. (19:79)
We will record where? In THE BOOK obviously
So whoever does good works and he is a believer, then his efforts will not be rejected and We will record it (kaatiboon) for him. (21:94)
We will record where? In THE BOOK obviously
This is Our record (kitaabana); it utters the truth about you. We have been recording everything you did. (45:29)
Similarly 69:19, 83:7, 83:18 refers to the same book.
This is THE BOOK that we cannot deny per 40:70.
As to "what we have sent our messengers with" in 40:70, it refers to all the books that were sent with the messengers (plural), including the quran. Unfortunately the only authentic remaining out of them now is the quran.
The mistakes in this post are all mine. Only the god knows the complete truth.