The Spy Game (Boxcar Children Mysteries)

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?