is it difficult to get?

Diroth open the entrance to cursed librery each 1 h inside you have 3 dk and you need to use bookcase till you get book but each use hit you

Like Frino said, every hour Diroth will open the staircase and if you are fast enough you can collect two-three books in 1 go.

Important to make ur path clear, kill the dark knights or lure them away.
Atleast kill the 3 below diroth before you click any bookcase as this sets the timer to close. (i think its random)

If you have a clear path, as savage said.. you can collect 2/3 book per go.