The urban landscape has long served as a canvas for muralists and street artists. Now, it seems, the city has also become something of a Post-it note for anonymous poets. In 2006, the designers Josh Kamler and Axel Albin noticed a new kind of marking cropping up across San Francisco’s Mission District: short, enigmatic missives from parties unknown. Ranging from the bittersweet (“I miss you”) to the nonsensical (“Mr. Seahorse knows some boys can have babies”), the jottings suggested the idea that, as Albin puts it, “someone is trying to tell you something.”

Calling this phenomenon “message graffiti,” the pair created a website, allowing users to post photos of similar notes from city streets across the world. Now they are collecting these messages in a book, a collection of such messages, called Written on the City. At once a design feast and a strangely affecting bit of populism, the book also gives the missives new voice. Mr. Seahorse, meet the world.