Death at Blenheim Palace / Robin Paige.

Enhanced descriptions from Syndetics:

In 1154, Marlborough Hall hosted a love affair between young King Henry II and a teenaged Rosamund, right under the patrician nose of his wife. To keep his mistress safe, Henry built Rosamund a house only reachable by labyrinth. But his efforts proved futile when he found her poisoned to death-and with her died the secret of the killer's identity... Now, in 1903, the Duke and Duchess have invited husband-and-wife scholars Charles and Kate Sheridan to Blenheim Palace-which stands now on the historical site of Marlborough Hall-for Kate is working on a book about the age-old scandal. As she tries to pull together pieces of the mystery, Kate must lend a hand when a household maid is kidnapped-and, not long afterward, the Duke and his mistress disappear. But Kate may have set aside her research too soon. For as the intrigue unfolds, it holds an uncanny resemblance to another, much older affair.