The Red Rose of Anjou (Plantagenet Saga #13) by Jean Plaidy aka Victoria Holt

The Earl of Warwick, known as the 'Kingmaker' had the power to make a king... and to unmake him.
When Henry VI becomes king, it is soon clear that he would be better suited to a quiet life than to ruling the country. Richard, Duke of York, is convinced that he would make a better king and has more right to the crown, and he will stop at nothing to claim it. But Margaret of Anjou, Henry's new French wife, is a formidable woman who is just as determined to keep Henry on the throne.
Most powerful of all is the Earl of Warwick, the kingmaker, and with his support of Richard of York the War of the Roses begins. When Henry VI lapses into madness and eventually meets his mysterious end in the Wakefield Tower, Margaret directs all of her ambition towards her young son, Passionate and impulsive she begins scheming for him, and in doing so dashes headlong into disaster ...