Kiss and Tell by Suzanne Brockmann

When Leila Hunt is swept off her feet by a ninja at a New Year's Eve costume party, she has no idea who the masked man is - but after he disappears into the night, she's determined to find him. When the clock struck twelve, what began as a friendly New Year's greeting quickly became the most unforgettable kiss Leila had ever experienced with anyone - including her absent boyfriend. But of all the possible suspects, Leila hardly imagines that the ninja is the same man who's teased, tormented - and secretly intrigued her - since childhood.