So... why didn't the demon just go to the monastery on his own? Why did he need to bring other people along for the ride?
Probable due to some sort of rules. The general reason in stories like this is demons are bound by rules set by God that limit them. In this case the demon had to be carried to the monastery. It just couldn't go there itself.
It's also possible it just didn't know where the book was and needed people who did know to take it there.
Why did they waited till there was only one book left to copy it? They could have did this before there was only five or ten of the book left.
The demon destroyed all of the other copies. The monks probably wised up to what was happening a little too late. Either that, or the demon works very fast and effectively; maybe they did copy it before, but the demon destroyed all of the copies until there was only one book left.
Don't forget, they didn't have photocopiers or even printing presses. They had to make their copies by hand. You can destroy a book a lot faster than you can make a new copy by hand.