this and i ripped down a huge amount of those porn posters that are hung in the phone booths
Those cards are a dying art!
Seriously, they are primarily a London phenomenon and are illegal now (though obviously they aren't going away). Some of them are great pieces of ephemera (right, Celtic?!). Anyway...allow me to plug a friend's book - it's only been out for about a year:
Tart Cards: London's Illicit Advertising Art
Caroline Archer
Mark Batty Publisher
It's available on Amazon
It's on the American Amazon, too.
Edited to add: Many people collect these cards, too!