The passport books look a lot like an actual passport on the outside. On the inside there is a page for each counrty in World Showcase. Each country has a location, usually in a gift shop, where a CM will stamp the country's page in your passport and write a greeting in the country's native language (with English translation, except in the UK and American Adventure). The stamps are logos representing the country.
The best place to get them is at the little shops at the entrance to World Showcase from Future World. Or, if you are entering WS from the Epcot resorts at the International Gateway entrance, you can get a passport from the gift shop just inside the gate on the left.