Games Workshop is always very short on time between announcement of an item and the item going on sale. We saw that with the new Eldar just recently, announced 5 days before sale.
This new supplement has a few more days on that, due for release on the 15th of June. There will also be three versions of this book upon release, a lavish full-colour hardback (GW don’t tend to make soft cover books anymore), a big fancy limited edition hardback and a digital edition (only for iPad and iPad mini, sorry Android users). There are only meant to be only 500 of the limited edition ones, so if you want it, get that pre-order in really quick. The digital version includes 360 degree views of models in it.
This book provides rules for making an Iyanden themed army, when used in combination with the Codex: Eldar.
The site is here, with informative video.