'Dune' is the first one that springs to mind if you want something that gives you good writing and absorbing characters, although it only has a Fog index of 7.9 according to Amazon, so I doubt you will be reaching for your dictionary (depending on you level of vocab of course). It's funny, but for SF set in space although there's a ton of books out there which are very well written and which have great characters, I can't think of a one which has had me reaching for a dictionary more than once or twice.

I would be interested to see if anyone knows an SF novel set in space which has a fog index of 12 or over.