Ceremony of Innocence (Windows & Classic Mac, Real World Multimedia, 1997)
You can download it here, or download it pre-configured to run on modern versions of Windows here.
An epistolary novel consisting of postcards whose fronts you have to solve in order to see the backs. Based on the first book in the Griffin and Sabine series, which you can digitally borrow here (plus here - chronological list here).












