This novel (published in 1971) just might be the fount of my fascination with criminals named Jack.
I'd forgotten how good this was -- especially for its time. The world on which it's set doesn't rotate: there are massive forcefields protecting the dayside, with a civilisation remarkably like our own, while on the darkside magic works, individuals are reincarnated and there are more feuds, vendettae and backstabbings than a barrelful of Borgias. All the darksiders are keen to rediscover 'Kolwynia, the Key that was Lost'. Jonathan Shade (dayside persona of infamous thief Jack of Shadows) has something that none of them have, though -- access to a newfangled dayside invention, a supercomputer ...
Jack is swashbuckling, romantickal and probably good-looking -- which goes with the territory -- and if that were all, it'd be an entertaining read. But there's a deal of philosophy in here, and the decisions that Jack has to make read quite differently to me now than they did in my teens.
Love the open ending, too.
edit: and am goggling at the Amazon Marketplace prices ...*clutches book fiercely to s(h)elf*