Innocent Blood by P. D. James

Adopted as a child, Philippa Palfrey had the best of everything. But, at eighteen, she wanted more. She wanted to know the identity of her real parents. Having always fantasized herself the illegitimate daughter of an aristocrat and a parlour maid, she wasn't prepared for the shocking truth. Philippa was even less prepared for her mother, and for their reunion... a rekindling of ties that would set in motion an event more terrifying than either of them could have imagined...