It was planted there by the Guerilla Poetics Project, whose website says:
Our mission is simple: Returning poetry to the people by subversively putting it into their hands.
In the past 26 months, GPP operatives have covertly smuggled 36220 poems into bookstores and libraries around the world.
I got these pictures off the website, but mine looks just like this one except that it has a number, which I entered into the 'found' area on the site.
If this underground movement appeals to you, you can become an operative, check it out!
