Master of Souls
I enjoy traveling by book both in time and place. I quite enjoyed "Master of Souls" a Mystery of Ancient Ireland book 15 of 16 by Peter Tremayne pseudonym for Peter Berresford Ellis a well-respected authority on the ancient celts. It features Sister Fidelma & is set in 7th century Ireland with a well thought out plot. I could see where some of the red herrings were leading me but was pleasantly surprised with the twist at the end.
More information on the series apears at the website The International Sister Fidelma Society.
Another excellent mystery series I recommend is the "Brother Cadfael" series of 20 books set in the 12th century in Shrewsbury, England & written by the late Ellis Peters pseudonyn for Edith Pargeter.