I was looking at pictures of my son as a preschooler today and remembered how much my son LOVED BLues Clues, the Nick kids TV Show, when he was 2. I know it's not a book, but one of the things I loved about the series was that it had this "mystery" theme, ever episode was trying to figure something out using a set of "clues". At the time my son was this age (12 years ago) one could get books and workbooks. At about the time my son was growing out of it Nick dumbed down the show, so what I remembering being impressed by was the original show with Steve. I have no idea if it's even still on TV, but I'm sure one can still buy the DVD's.
