I'm pretty sure that the first mystery I ever read was a book called The Key to the Treasure by Peggy Parrish. In it, three siblings who are spending the summer with their grandparents endeavor to find the solutions to several mysteries hidden in artifacts that their uncle hid when he was a little boy. I haven't read it in quite some time, but I remember how much I enjoyed the fun of solving the puzzles that the uncle hid. And so began a lifelong interest in mystery novels.
The Secret of the 14th Room has a little bit of everything: