The Highlander books are a series of romance novels set in medieval Scotland. While each of the novels are stand-alone, they involve similar story lines (several of the books feature time-travel between modern times and medieval times) and recurring characters (most notably, Adam Black, the Faerie King's fool).
In certain respects, the Highlander series is a prequel to the Fever series in that they introduce the world of the Fae, as well as the MacKeltar family, who feature in Faefever, Dreamfever and Shadowfever.
The Fever World series is a new trilogy set in the Fever World, featuring Dani O'Malley, a rebellious young sidhe-seer, as the main protagonist. In addition, the books will feature other characters from the previous series, including Christian MacKeltar, Ryodan and the mysterious 'Dancer'. Moning has stated that while each of the books will be a 'stand-alone mystery', they will contain 'larger plot arcs' which will answer many questions left unanswered in the Fever series.