Farsight, a Tau codex supplement announced in the July White Dwarf is on its way very soon. The Black Library released this image earlier, and its now very much everywhere by now. Farsight is the second 40k codex supplement release in 6th edition, and I feel that this one will be just as popular as the first.
Now please realize that this will be released digitally first, and if the release schedule of Iyanden is followed, a hardcover book should be a few weeks after. That would very much make it arriving early in August, if not the first release date of that month; August 3rd.
Link to the Black Library Digital Releases
http://www.blacklibrary.com/games-workshop-digital-editions