«Love and The Marquis» by Barbara Cartland

Now she is sixteen and a half, the beautiful young Lady Imeldra is eager to leave school and resume her exciting life with her father, the Earl of Kingsclere. He is renowned in Society as something of a 'man about town', but when he goes on his frequent extensive travels round the world, he always takes Imeldra with him and she just loves being with her father.
She is dismayed when he announces that his rather decadent lifestyle is not conducive to the necessary task of presenting Imeldra to Society and at Court and finding her an appropriate suitor for her hand in marriage.
And he is intending to send her to live with her strait-laced and disapproving grandmother, where she will lead a very dull and formal life
Rebelling, she enlists the help of an old family friend, William Gladwin, who is currently building an ornate orangery at the neighbouring estate of Marizon and passes herself off as Mr. Gladwin's grand-daughter so that she can stay at Marizon and perhaps explore the house that is renowned for its superb collection of pictures by great artists.
Although the house and its estate is incomparable in its beauty and grandeur and its brooding cynical Master, the Marquis of Marizon, is equally handsome, Imeldra has the distinct feeling that something is amiss and that the Marquis is hiding some dark secret from everyone around him including her.
And, seeking to know more of the mysterious Marquis, she finds to her surprise that she has fallen in love.