The Spy Game — Gertrude Chandler Warner

When the Boxcar Children visit a neighbor's house, a mysterious old wedding photo and a riddle lead them on a “spy game” devised long ago by the original owner of the house. When the children hunt for clues they discover amazing things, like a stone path whose pieces fit together into a jigsaw puzzle! But soon the children begin to suspect that there's a mystery inside the mystery. They read about a similar puzzle in one of their new Detective Club books, and they meet a woman who looks eerily like the bride in the old wedding photo. Just who is behind this spy game?