In most establishments, there will be a sign posted that a "Libro de Quejas" (complaints book) is available. The toll plazas have them, bars and restaurants have them, etc etc. I've never seen such a thing anywhere else in the world. What is the function of these books? Are they just maintained by the establishment so that someone who is ticked off can vent on paper? Is it a government book that is monitored by the authorities? Do they really exist?