Contrary to another reviewer, I absolutely enjoyed this book (then again, it is a Heyer novel). Because of its light-hearted romantic approach in the midst of "the devil's own scrape," this is one of my first recommendations to readers who I am introducing to Heyer.
Poor Cardross is in love with his wife, but doesn't know how to show it so very well. To make things worse, it's just not the thing to sit in your wife's pocket, and the servants always seem to walk in whenever he wants to display affection.
Nellie, always aware of the fact the she had to marry Cardross/his money since her family didn't have a feather to fly with, finds herself equally attached to her husband, but wondering how she can show him that it's not his money she loves (her pockets-to-let spending isn't very convincing). Due to well-meaning but careless comments from his younger sister, Nellie is aware of his past "liaisons", and figures that his current reserve may very well be due to the same.
The difference in the ages ( 30's vs. 19) accounts for much of the misunderstanding. He's a Man of the Town and she's still a bit of a green girl. She is not the cleverest of heroine's but very likable. He is not the completely rakish fellow one loves to laugh at, but the reader feels that he is indeed the strong and wise hero able to smooth over all the heroine's mistakes.
Her unwise choices create many doubts in her husband's mind that he is trying to overlook. His overlooking makes him reserved, causing Nellie to fear his final rejection. Her foolish but generous use of money has created a bit of tension, and sets them up for an amusing conflict that is carried through the book, with laughable little twists-and-turns until the end.
Also, the secondary characters - Cardross's friend and younger sister - add tons of color and wit to the story.