What's the Name of That Book??? discussion

It's most likely a children's book, but I remember one of the characters, who I think is an adult and a sort of helpful older fatherish figure to the main characters is locked in a dungeon and escapes by gnawing off the ends of his fingernails and using them to make a lock-picking device.

When did you read it?

Sometime within the last eight years

The Mysterious Benedict Society by Trenton Lee Stewart? Milligan (the children's guard) does this in a flashback scene.

That sounds right. Thanks!