Experimental Mode is best for using console commands and the spawnable item list for testing.
Click the local files tab and you should see an option to verify integrity of game cache.
Wait for this to complete and then resume your download.
I have 16 GB free on my Steam installation disk and 60 GB free on my system disk, so, naturally, I expect everything to work in no-brain mode.
However, halfway through peacefully downloading the patch in background Steam throws a "Not enough free disk space" error.
I've currently my DF: C installed with all the released books present.