This new Book will continue the Epic Story, with a major quest arc & raid, and new cosmetic upgrades, among other updates.
The new screenshots depict three new Skirmishes that will come with Book 3.
Book 3, Echoes of the Dead - New Skirmishes:
Three new Skirmishes erupt from the depths of Moria. Beyond the barriers of the 21st Hall, in the nighted gulf of the Deepway, and along the Way of the Smiths, defend the brave dwarves of the Iron Garrison from the hordes of Mazog. These Skirmishes are waiting for you to test your mettle and hone your skills whenever the call of battle sounds.