I enjoy historical romances and especially those set in the Highlands of Scotland.  I am looking for recommended authors especially those who write series books.

Monica McCarty...I've enjoyed everything I've read by her.  Also, Karen Hawkins - she can be hit or miss in my opinion.  Other authors have written Highlander/Scotland novels that write Regency era stuff...like Sabrina Jefferies & Julia London.  So while the entire series may not be all Highlanders...there are some thrown in.  This is what comes to mind....I'm sure there are many others.  Good luck on your search. 

Karen Marie Moning

Karen Ranney

Kimberley Cates

Cathy Maxwell

Sue Ellen Wellfonder

Terri Brisbin

Lois Greiman

Patricia Veryan-Golden Cronicles trad Regency-in particular #2 Journey to Enchantment

I second Karen Marie Moning... awesome Highlanders series.  Also Monica McCarty.

Kresley Cole (If You Desire, If you Deceive, If You Dare)

Kinley MacGregor

Julia London

If you want to branch out into contemporary, Donna Kauffmann & Janet Chapman write some warm-hearted, sexy Scottish stories.

Hannah Howell has a large series of Highland books.

Arnette Lamb wrote some good ones.  Julie Garwood.  And of course Diana Gabaldon!

Oh!  I forgot Arnette Lamb.  Loved her books. "Highland Rogue" in particuliar-then the others in the series about his daughters. 

Also, Beatrice Small and Amanda Scott.  Alot of Scott's almost tread into the historical fiction variety now that I think about them. 

Others-Sarah Gabriel-historical fiction as well; as well as her pseudos Susan King and Susan Frasier King

You must read Susan King -  The Stone Maiden (Maiden Trilogy, Bk. 3)
 and also  The Angel Knight and  Laird of the Wind

Monica McCarty

If you are willing to spend the money on them, I HIGHLY recommend getting the audiobooks for Karen Marie Moning's Highlander series.  I'm listening to my last one now.   Phil Gigante's voice makes me melt.

Angie Kay - "A Case of Nerves" - it's on my bookshelf!