39 Clues #5: The Black Circle by Patrick Carman

A strange telegram lures fourteen-year-old Amy Cahill and her younger brother, Dan, deep into Russia and away from the only trustworthy adult they know. Signed with the initials NRR, the telegram launches a race to uncover a treasure stolen by the Nazis and the truth behind the murder of the last Russian royal family. All too soon, the treasure hunt starts to smell like a Lucian trap. But the bait might just be irresistible...what will Amy and Dan risk to find out what really happened on the night their parents died?