From the Wikipedia page 
The Clock Without a Face is puzzle book by Eli Horowitz and Mac Barnett with illustrations by Scott Teplin. It was published in 2010 by McSweeney's. The plot of the book revolves around the theft of 12 jeweled numbers from the face of "the Emerald Khroniker," a cursed clock, and with clues to the thief's identity and the whereabouts of the numbers are included throughout the text and in accompanying pictures. The identity of the thieves is revealed on the last page, but the location of the loot is not. The authors hid 12 actual emerald-studded numbers (crafted by Anna Sheffield) in locations around the United States, and readers are encouraged to search for them using the clues in the book.