The Foglios describe it as "...gaslamp fantasy with adventure, romance and mad science." It's set in a (alternate-history) steam-powered, Industrial-Age Europe, where unique individuals have super-human abilities with technology and invention. These "sparks", as they're called, variously employ their abilities for good and evil... resulting in grand tales of conflict and heroism.
It's a great series of books, with particularly beautiful establishing shots and backgrounds. (The kind you look at for ten minutes, soaking up all the detail.) If you haven't read it before, dive on in online.