The Caledonians, Paperback

Scottish history master Mr Petrie has the gift of eternal life. Working for a group of mystical superior beings, his time-travelling missions land him in all sorts of death-defying scrapes and encounters, sometimes with famous and ruthless people. To help him in his dangerous work, he's told to find a young apprentice. Duncan Dewar could be a candidate but has his own secrets too, and without realising it, their lives are indelibly linked.