I'm fairly sure 559 is the standard mountain bike size, so you should be able to get narrower tyres easily enough. You can get them with a road tread too, something like this http://www.chainreactioncycles.com/Mode ... elID=69985 (that's an example, not a recommendation btw)
I got a set of tyres in 26 x 1.75 which were like slick MTB tyres but (my head hurts) they were too deep to fit the frame - they were 40's and i need 30's i think - any suggestions?