The Castle of Otranto: A Gothic Story by Horace Walpole (1764): A giant helmet fell from the sky, the princess has escaped through the trap door, everyone is terrified of the giant ghost which has manifested in the gallery and just who is the mysterious Alfonso!? Set in Medieval ish Italy this is a fast paced adventure of gothic proportions and you can download it for free via iBooks
The Daughter of Time by Josephine They (1951): This is one of my favourite books of all time. When Inspector Alan Grant of Scotland Yard falls down a trap door, he is confined to his hospital bed and grows incredibly bored. When his friend suggests he try and undertake some academic investigation, she brings Grant a number of portraits of historical personages who have some sort of mystery or unsolved crime attached. Grant prides himself on being able to judge someones character from their face and becomes intrigued by a portrait of Richard III who despite being known as a notorious murder, to him has the ‘face of a judge’. With the help of a British Museum researcher he starts an investigation into history; what kind of a man was Richard III and what really did happen to the Princes in the Tower?