The book does look good but not really worth the cost in my opinion. I'd say the two main reasons to buy the book is you are going to run a full Ghost Army and the new rules in the book will make it a stronger build. Or if you are into fluff and want some stories and background to read. Other then that it's pretty much some pictures and a few missions you can play.
A breakdown of what you get:
Rules - Two pages of rules that do offer some benefits for running a full Ghost Army. But there isn't anything in there that really stands out as amazing to me and the real big let down is no Iyanna rules. I know Iyanden already has Yriel but Iyanna is as big a part of the Craftworld's history and would probably be used more. What would an Iyanden army really use more? A Spiritseer or an Autarch? Not really a hard choice there.
Story / Fluff - Ok, so this book is written by Mat Ward, a mostly fluff book by Ward? Yuck. Too bad he didn't give the book a broken new unit for the Eldar to exploit haha but he did work his normal fluff magic. Most of the background stories are ones you've already heard. The Fall, the Nids attacking but there are a few new stories in it. Sadly I wasn't really impressed with the creativity of them. One being Iyanden being cut off from Exodite worlds by Warpstorms which eventually clear up. Umm.. that for one sounds just like Mankind at the start of the Great Crusade and two Warpstorms don't affect the Eldar all that much. The webway skirts the real and the warp and space is pretty big. They would exit the webway and just fly around or take another webway exit and fly there. The stories of how awesome the Hemlock Wraithfighter are make me laugh but I guess if you could field 80 of them like in the story they would be awesome.
Pictures - The pictures are cool and some of the iconography is useful. Most of the pictures are the standard you've seen before but if you are planning on playing and painting an Iyanden Army it could be helpful.
