Between us, Husbit & I own a fair few roleplay books, and we've played maybe two thirds of the settings. If you go further and break it down to source books, that probably gets to be even worse odds - especially if you take him out of the equation and only look at me.
But of all the books, of the ones that are actually mine, I think the one I've had longest and done nothing with is Abney Park's Airship Pirates. (I say mine, I bought it for Husbit but he wasn't all that interested so I claimed it for me ;-) ).
I bought it because I like the band Abney Park and I was entertained by the back story they created to justify their change from goth to steampunk and the concurrant change in line up. It feels like the start of a roleplay story: their plane was caught in a freak accident with a time travelling dirigible (the Ophelia). They combined with the survivors of the crash to become airship pirates (see Wikipedia and their website).
The RPG in a far future that's pretty broken, with most people living inside a huge walled city where rebels and dissenters are thrown in the 'change cage' and there's dark things going on. Outside, there are many dangers but also more freedoms. You've got people living in cloud cities, you've Neobedouin nomads, and of course the airship pirates themselves.
I'd very much like to play or run it one day, but for now I'm happy just owning it and enjoying the setting.
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