The passive, pretty, and sweet-hearted Imogen and the plain, older but very effective Blanche fight for the affections of Distinguished barrister Evelyn Gresham--a battle that should be easy for Imogen to win, as she's Evelyn's wife, but it doesn't turn out that way. The story is told almost entirely from Imogen's point of view, and I found it impossible not to like her as a character, despite her comparative uselessness (it helps that Blanche is useful in ways that involve shooting woodland creatures and hectoring knocked-up schoolgirls).
Lots of reviewers compare this to Jane Austen, and it does involve rich-ish English people and marriage, but plot-wise that's where the similarities end. Jenkins is much more sympathetic to both Imogen and Blanche than Austen would have been--Austen would have given Blanche both barrels, I think, and couldn't help but have played up Imogen's blind foolishness.
Possibly the only story I have read in which the Cinderella story serves as a B-plot, and begins with the protagonist carelessly making out with the Prince Charming figure.