Gravity and Normal Force

I understand that gravity and the normal force are NOT an action reaction force pair. But this leaves me with the question of what actually causes the force to be exerted downwards on the table by the book to generate a normal force on the book acting upwards ?

Thanks.

It's the other way round, really. The best way to think about it, is first to consider the forces on the book.
So, considering the book: It has its weight acting downwards on it, which, in the absence of any other force, will cause it to accelerate downwards. (Acceleration due to gravity)
If that was everything then it would indeed just fall. So let's put a table in the way.
To stop the book falling you have to apply an equal and opposite upwards force to its weight so that the resultant of that force and the book's weight is zero. Then you get zero resultant force on the book and it remains at rest.
So, the upwards force that is applied to the book is the force from the table. If the table wasn't there, there would be no upwards force.
So first comes the table's upwards force on the book. The Newton's 3rd Law reaction to that is the force of the book on the table.

So what you are saying that for book (b) and table (t) :

F t on b = F b on t = F weight of book ?