Well, my co-authored book on gladiators beat my solo-authored book on ninjas to 500 copies.
GURPS Dungeon Fantasy: Ninja came out August 2010 and has racked up 465 sales to date.
GURPS Martial Arts: Gladiators came out earlier (June 2009) and it's gotten to 501 sales to date.
So yeah, over a year's difference and only 36 more copies sold, but still. I'd have expected the ninjas to sneak up on the gladiators and overtake them. But Gladiators has sold steadily the whole time. Either way it's nice to have another book over the 500 mark. I just expected a different one to be next.