I just logged out of a rather bloody match of CS and saw on my Steam client that I had a download. I didn't pay much attention at the time because every other day it seems like TF or CS gets a download or update of some kind.
Imagine my surprise when I see a pop-up telling me that Eschalon Book I has finished downloading!
I booted it up and sure enough - it launches again!
PS: Now hopefully Book II gets a patch soon....