One of my favourites that starts out difficult but you become fluent in by the end is Banks' "Feersum Endjinn". I love seeing people's facial expressions on first attempting to understand Bascule, or read it "normally".
Eh. The only thing I remember offhand of Bascule's orthography is "Ergates thi ant", although for some reason the book is the rare one where I've effortlessly memorized the names of many protagonists (I'm not very good with names, both IRL and in literature.)