From Wikipedia:
I Spy is a children's book series with text written by Jean Marzollo, photographs by Walter Wick, and published by Scholastic Press. Each page contains a photo with objects in it, and the riddles (written in dactylic tetrameter rhyme[1]) accompanying the photo state which objects have to be found.
Although the first I Spy book contains unrelated pages of still life pictures, subsequent books are more thematic.[2]
Several video games based on the I Spy books are available for Windows PC, Nintendo DS, Wii, iOS, Leapster, and Game Boy Advance, including I Spy Spooky Mansion, I Spy Treasure Hunt, and I Spy Fantasy. These served as early examples of an increasingly popular hidden object game genre.