The Caldera by John Flanagan

The Brotherband series book 7. Random House Australia, 2017. ISBN 9780857980137
(Age: 11+) Highly recommended. Themes: Survival, Rescue, Kidnapping, Piracy, Team work, Sailing. Charged with the task of rescuing a kidnapped Emperor from a far-flung volcanic island fortress, the Brotherband (led by the extremely capable leader - Hal) are again able to demonstrate their highly honed fighting and sailing skills. This crew of mostly young Skandians is able to sail the small craft, the Heron, with fearless expertise and superb teamwork. Despite violent opposition from pirates and the elements, yet again the crew take on a mission that will push them to the brink . . . but a life depends on them acting quickly and putting the rescue mission before their own safety.
John Flanagan writes with great flair to produce an action-packed adventure that all readers will enjoy for its adrenaline rush and wonderful introduction to the world of skilful sailing.
Note: the book also includes a bonus Ranger's Apprentice short story. This is a surprise gift at the end.
Highly recommended for readers aged 11+ (even if you have not read any of this series before book 7)
Carolyn Hull