Twenties girl : a novel / Sophie Kinsella.

Enhanced descriptions from Syndetics:

Lara Lington has always had an overactive imagination, but suddenly that imagination seems to be in overdrive. Normal professional twenty-something young women don't get visited by ghosts. Or do they?
When the spirit of Lara's great-aunt Sadie-a feisty, demanding girl with firm ideas about fashion, love, and the right way to dance-mysteriously appears, she has one last request: Lara must find a missing necklace that had been in Sadie's possession for more than seventy-five years, and Sadie cannot rest without it. Lara, on the other hand, has a number of ongoing distractions. Her best friend and business partner has run off to Goa, her start-up company is floundering, and she's just been dumped by the perfect man.
Sadie, however, could care less.
Lara and Sadie make a hilarious sparring duo, and at first it seems as though they have nothing in common. But as the mission to find Sadie's necklace leads to intrigue and a new romance for Lara, these very different twenties girls learn some surprising truths from each other along the way. Written with all the irrepressible charm and humor that have made Sophie Kinsella's books beloved by millions, Twenties Girl is also a deeply moving testament to the transcendent bonds of friendship and family.