The Tower Treasure — Franklin W. Dixon

In the first novel of the classic children's mystery series, the Hardy Boys' quest for stolen jewels leads to unexpected danger and adventure.
 
Frank and Joe Hardy can't let their friend's father take the fall for the jewels stolen from the Tower Mansion, despite how guilty he looks. But once the teens find the real thief, a master of disguise who had everyone fooled, he dies, leaving behind a cryptic message about where the loot is hidden. Now the brothers must set out on a hunt to find the treasure, which leads them to an old tower -- and a trap -- from which it appears there is no escape . . .