Realm of the Ice Queen — Lara Nance

Book Four of the Airship Adventure Chronicles-

Belle and Rett's adventures are not over.
While Rett is involved in rebuilding the destruction caused by Dr. Krakov in the capitol city, the queen calls on Belle for a vital mission.
She sails off to Icelandia to negotiate a treaty with the eastern countries, only to find the queen of that frigid land is hiding a secret that threatens to derail the vital progress of peace.
Rett unwittingly uncovers a threat beneath Aereopolis, more dangerous than the attack of the mad scientist. This discovery sets off a chain of events that unleashes a century-old menace.
Once again, Belle and Rett must count on resourcefulness, courage, and old friends to escape peril and save the day.