The Scottish Prisioner by Diana Galbadon

As Mrs. Galbadon states on her website, this is one of the "little" book in between the "great big books".  If you have read the Outlander series this story fills in a few things, if not it is a stand alone novel.
Lord John Grey needs the help of Jamie Fraser, aka Alex Mackenzie, to investigate part of the evidence of a court martial.  Things are never quite that simple of course but it does get the story rolling.
Filled with intrigue, plots, counterplots, spies, a sprinkling of Jacobites and several ah-ha moments.
Very enjoyable read which I highly recommend.