This is one of the books of 2009 that I so anticipated reading. I loved The Time Traveler's Wife (my review here), and I'd read good things about Niffenegger's second novel.
Happily, I was not disappointed. Her Fearful Symmetry is the story of two sets of twins, Elspeth and Edie (the first generation), and Julia and Valentina, the daughters. When Elspeth dies, she leaves her London apartment to her nieces, and then comes back to haunt them—affectionately. Sort of.
Along with the apartment come an assortment of inhabitants, including Elspeth's long-time partner, and a lot of secrets. While Julia and Valentina try to figure out their own identities and sort through secrets, their own relationship begins to fall apart.
Her Fearful Symmetry reminded me a lot of Diana Setterfield's The Thirteenth Tale (my very short review here), with a satisfying inclusion of ghosts and cemeteries. The last few chapters of Her Fearful Symmetry fell a little flat for me, but I didn't care too much. Niffenegger is a fantastic storyteller. If the idea of time traveling put you off of The Time Traveler's Wife, I'd recommend starting with Her Fearful Symmetry. (And then—go back and read the former, please!)