War of the Dragon by Jessica Drake

When Zara and Lessie finally make it back to Elantia, the last thing they expect is to find that nearly all the dragons have gone missing.

Worse, the Zallabarians are cutting a swathe through the country, mowing down the Elantian soldiers as they head straight for the capital. The only chance Zara and Tavarian have at stopping them from destroying Elantia lie in the recovery of the missing dragons.

But finding a horde of dragons that have vanished into thin air is no mean feat, especially when it involves heading straight back into enemy territory. And when Zara stumbles onto a secret regarding the next piece of the dragon heart, she must make a terrible choice between saving all of dragonkind, or saving the world…