I signed up for one already and am ordering another today.
These are on the 18-count boxes and maybe other Kelloggs products as well. You buy the box, then go to Kelloggs and enter a special code from inside the box.
I've ordered Finding the Titanic and April Fool's Surprise. They say it takes 4 to 6 weeks to get them.
Treats and a free book? Double score!