Price of a Princess — Nigel Tranter

After young James the Third's accession to the Scottish throne, the
ambitious Boyd family of Kilmarnock seized power in a bloodless coup.
Mary Stewart, James' eldest sister, was at first unwilling to marry
Thomas Boyd, future Earl of Arran - but she had no choice.

Eventually, however, she learned to love him, and when he was sent to
the Danish court to negotiate with King Christian, she discovered a
unique talent for diplomacy. In exchange for Princess Margaret of
Denmark marrying her brother, Princess Mary at length persuaded the
Danes to hand over the islands of Orkney and Shetland to Scottish
dominion.

But when the fortunes of the all-powerful Boyd family took a turn for
the worse, Mary was to find herself in an extremely awkward and
dangerous position...

A riveting tale of romance, treachery and heartbreak, set in 15th
century Scotland: the story of Mary Stewart, eldest sister of James III
of Scotland and her part in making Orkney and Shetland part of Scotland.