I have no idea which book it might have been because Terry Pratchett usually brings out at least two books a year. If you could tell us what year it was and what time of year, we might have more to go on.
But as One Man Bucket says, the only way you'll get a signed book now is by ordering from the link. And Terry doesn't usually do dedications any more.
Look, you've got it all wrong! You don't NEED to follow ME, You don't NEED to follow ANYBODY! You've got to think for your selves! You're ALL individuals!
Yes! We're all individuals!