See the thread on WOLP postcards further down the board, and Jasper's
giveaways explaination.
Basically, Jasper sometimes sneaks postcards into copies he's signing, as a sort of Easter egg for the owner to discover. And if you go to a signing in person you are almost certain to get one.
I'm guessing you have a US hardback of LIAGB, in which case the bookstore must have asked Jasper to sign some extra books for them, and he slipped the postcards in at the same time. So you got a happy surprise.
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"I've often said that the difference between British and American SF TV series is that the British ones have three-dimensional characters and cardboard spaceships, while the Americans do it the other way around."
--Ross Smith