The 27-year-old singer reportedly had her security team boosted as a precautionary measure ahead of the event, after being attacked by a crazed fan at her last signing for her autobiography in 2009.
The star, who's been romantically linked to One Direction's 19-year-old Liam Payne, wore a black and white prom-style dress with sheer sleeves and neckline and a pair of Mary Janes as she greeted fans at the store in Central London. She wore her hair pulled back into a super high sleek ponytail.
Lewis performed upcoming single 'Trouble' on The X Factor's first live results show two weeks ago.
Copyright: Ken McKay/Thames/Rex Features