Getting Rid of Matthew by Jane Fallon (2007)

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Unexpectedly charming novel about a woman whose married beau leaves his wife for her and she finds out that's not what she wants after all. She hatches a complicated plan to get him off of her hands which includes making up a new persona to befriend his wife and convince her to take him back.  Surprisingly good character development, this novel successfully avoids many of the cliches in chick lit. Very enjoyable!