Lord John and the Private Matter

This is a book you'll want to read after the 3rd Outlander novel (though I waited until after the 4th) as events take place during Jamie Fraser's time at Helwater. While Fraser is mentioned a few times in the novel (as John is still infatuated with him) it's far enough removed from the Outlander series as to not be repetitive, but still reflects back on past events.
John takes us through the worlds of English military, London's seedier (and not only of brothels, but the less spoken of molly-walks as well) side, and on a great mystery worthy of a Holmes novel all with his brand of charm and wit.
Lord John Grey and mid-1700's London are infinitely more interesting than being inside Claire Randall-Fraser's head, wherever it may be at the time, all the time. He's definitely a person I'd want to know personally.