All of Caroline's books can be read alone, but if there's a strong overarching plot to the series, you'll enjoy them more if you read in series order. Sometimes, however, secondary characters from one series go on to get a book in another series. This is the suggested reading order if you're the type of person who gets twitchy about not reading those secondary characters in order.
The Lass Who Lost a Shoe* The Lass Who Kissed a Frog The Lass Who Loved a Beast The Scholar and the Scot The Doxy and the Duke The Duke's Virgin Sister The Earl's Brazen Widow The Rake's Insatiable Bride The Sinner's Tempting Captor The Polka-Dotted Bathing Costume (novellette) The Duke's Deceitful Governess ...and then the rest of the Surprise! Dukes
*The hero in this book appears as a secondary character throughout the Everland Ever After books, but since those are a different genre (historical western) and a different heat level (no spice), AND a different comedy level (really, Caroline, why did you even bother linking them?), it's not really necessary to read any further back.
Medieval Scottish RomComs
A Scot Mess Scot on Her Trail Getting Scot and Bothered In Scot Water Drop It Like It's Scot Scot to the Touch Scot Under the Mistletoe (novella) Too Scot to Hold Reindeer Romance (novellette) Plaid to the Bone Not Half Plaid Ye Give Love a Plaid Name Don't Give a Damn About My Plaid Reputation Plaid Case of Loving Ye He Kens if Ye've Been Plaid or Good (novella) Plaid Attitude
Medieval Scottish Adventures
The Sinclair Hound The Mackenzie Regent The Sutherland Devil The MacLeod Pirate The Laird's Yulebringer (novellette) The Bruce's Angel (novella) The Highlander's Angel The Laird's Angel The Thief's Angel The Pirate's Angel