Duty and Honor - D. L. R. Frase - Bog

Although Aryk was born an Islander, he washes ashore at a young age in the Flatland Kingdom, the only survivor of a shipwreck. King Konr raises Aryk as his own son. Years later, after a severe summer and serious drought, Islanders are detected making unannounced landings in Flatlander territory. King Konr has no knowledge of the Islander military strength and can only assume the worst by such behavior. In an effort to discourage war, he secures the betrothal of the daughter of the Highland king and an alliance he believes will be strong enough that the Islanders would not dare attack.
Aryk is given the duty of leading a party to the Highland Kingdom to escort Konr's betrothed to the Flatland Kingdom, using a map provided by Konr's couriers. As the journey becomes increasingly difficult, this map becomes more and more suspect and the couriers who provided it are assumed to be Islander spies attempting to stop the wedding and alliance.