The Woodville Wench by Maureen Peters (Fontana, 1973)

Picked up at a local school fete:

 

The King's Wife...

Determined to rise above the poverty of her upbringing, Elizabeth Woodville uses her great beauty to win a place in the royal family.

Edward Plantagenet

the handsome young King, risks everything to marry her.

Cecily, of York

the King's mother, welcomes her as a daughter

Richard of Gloucester

Edward's brother, offers her protection when his own life is in jeopardy.

Considered an upstart by many, Elizabeth reigns supreme during the bitter years of the Wars of the Roses.  But behind the brilliant mask is a cold and lonely woman, shivering in the chill of her husband's waning passion, fearful of what lies ahead...

This is an example of what I think of as "cleavage history", where the author takes a real woman from history and writes a wildly romantic fictionalised version of her life.  You can usually spot these by the models on their front covers who generally wear some kind of pseudo-historical garb complete with a deeply plunging  neckline.

In this case our heroine (who seems to have picked her dress up at Biba and who is sporting some very 1970s blue eyeshadow) is Elizabeth Woodville and the story the author is telling is the story of the War of the Roses.

The other giveaway for a "cleavage history" novel is the choice of adjectives on the back cover, which often includes words such as "passionate", "untamed" and "determined".  The Woodville Wench may not be passionate and untamed, but by Jingo! she's determined.