I just began playing a few weeks ago, but I've heard nothing of Umbriel (flying city from the commissioned book).

Has anyone seen anything in-game about it?

There is probably one or two but for a flying city it's not really mentioned a lot...

Hmm, that's a shame. I really enjoyed the books.

I believe the tree in the bandit camp near Whiterun (the source of Sleeping Tree Sap) is from Umbriel. But other than that, I can't think of any.