We have been wondering when this would go live, and its been hard to find, or perhaps I have just missed it. According to Games Workshop Digital, pre-orders are going live October 19th, thats right, Saturday!
This the second Codex Supplement to reach print copy. (wishing the gap would close), and here is what was said by Games Workshop.
via Sarah at Games Workshop Digital Editions
Regarding the Farsight Enclaves Hardback Release
I'm told it will be on sale this Saturday, the 19th of October from http://www.games-workshop.com/