The Regency period in England is technically the years 1811-1820 when Prinny--the Prince Regent who became King George IV--ruled because his father, King George III, was mad. It was a time of great change--the Industrial Revolution was well underway; the Napoleonic Wars were ending. (Waterloo was fought in June 1815.) And it is a favorite setting for historical romances.
When I started writing my first book, The Naked Duke, I knew I wanted to set my stories after the Napoleonic Wars--I'm just not a fan of wartime. So the Duke occurs in 1816. The second book, The Naked Marquis, happens a few months later. And the third book...well, the heroine of The Naked Earl is Lizzie, the Naked duke's sister. She was only 17 in the Duke, and while that would be a perfectly acceptable age to marry in Regency times, it was too young for my modern tastes. She had to grow up--so that pushed The Naked Earl off to 1819. Once I had this basic timeline established, I worked forward or backward depending on who the characters were in my new stories.
So here is the rough chronology of the books. And don't worry--they don't have to be read in any particular order. I didn't actually write them in the order they "happened."
1816, roughly concurrently: The Naked Duke, The Naked Baron, and "The Naked Laird" in Lords of Desire
1816, a few months later: The Naked Marquis
1819, roughly concurrently: The Naked Earl, The Naked Viscount
1820: The Naked Gentleman
1821, February: "The Naked Prince" in An Invitation to Sin
1821, April: The Naked King
Tomorrow, how the Naked characters connect.