And, there's also Nobilis: Antithesis, Minibook 1i, which I've just started. It looks like it has all the guidelines for creating Excrucians. Yay!
Also, as the Hitherby site shows, there's The Fable of the Swan, which I think is a Nobilis novel. I'll know for sure after I've read it. There's also An Unclean Legacy, which I'm trying to figure out if we already own. I've not yet come across it, but I've only just finished re-cataloging the bedroom. It might be in the parts of the study or the living room that I've not yet gotten to.