Midwinter by John Buchan is a historical spy novel and it's a good book.

The story is set in the time of Bonnie Prince Charlie, more precisely during the Jacobite rising of 1745/46, and its protagonist is a captain of the Scottish army travelling through England to join his Prince in Scotland. On his way he realises that his assumed friends are actually his foes trying to get rid of him with all means because they betray the Jacobite cause. He is helped by a not yet famous Samuel Johnson and a mysterious man called "Midwinter" rescues him ever again from almost certain death.

I wrote a long review of the novel on my main book blog which you can find following the link to Edith's Miscellany.