Firebound

For the first time in her life, Spark has friends and has cobbled together something that's starting to look like a family in her new home under the dragon city. But she's never felt more alone. Or under more pressure. It's only a matter of time before Loch and the ice dragons take revenge and attack the dragon city again. Spark throws herself into spellcraft as the dragons' only hope. But there's no way she can do it alone. And then there's the matter of having mere days to rescue a stolen dragon egg. 

In order to save the dragons, she needs her mother and her grandmother at her side. First, Spark must delve into the most dangerous kind of magic while learning that rebuilding a family isn't easy.

While this book is about grief and isolation and loneliness, it's also about queer joy in the darkness. It's about family and trust and healing. It's about being the light that drives away darkness. It's also the most like HTTYD of the entire series.