Sly Cooper: Those rats got pegged.
Bentley: Raleigh appeared to have booby-trapped this entire place.
Sly Cooper: I'm going to enjoy this.
― Sly Cooper and Bentley[src]
Upon entering Raleigh's library, Sly came across a booby-trapped rug, where two rats got pegged by two dart machines. Bentley surmised that Raleigh may have booby-trapped the entire place. Sly quickly thought of using the barrel in front of him for protection, though Bentley was not so positive about it. However, Sly's plan was full-proof, as he managed to sneak his way through and steal the treasure key.
There are 30 clue bottles to collect in this level, and the vault is located on the second floor, on the other side of the bookshelf with the ladder, right after the first two searchlights. The combination to the vault is 2-4-2.
With the help of a straight edge and compass the answer was simple. Dial in 2-4-2.