[The book] features new, never-before-seen artwork, full-colour miniatures galleries (including 360ยบ images in the digital version) packed with dozens of miniatures painted in the colours of Craftworld Iyanden, a hefty 32 pages of background about the Craftworld and its people, a detailed timeline of important events, colour schemes and iconography for the different Ghost Warrior houses and much more besides. If you're a hobbyist that loves to immerse yourself in the lore of Warhammer 40,000 then this book is definitely for you.
Codex: Iyanden also features a host of new rules which, when combined with the army list in Codex: Eldar, allow you to create a dedicated Iyanden-themed collection. Using the rules in Codex Iyanden allows you, for example, to take a Wraithknight as your Warlord. If that's not a fearsome prospect for your opponents to face then I don't know what is. There are also Iyanden-specific Warlord traits and the option to take a council of Spiritseers, plus three new battle missions and five scenarios to fight with your army. There are even stratagems for Cities of Death and Planetstrike for you to use on an unsuspecting enemy.