The Touch of Twilight (Signs of the Zodiac #3) by Vicki Pettersson

Made up of both Light and Shadow, Joanna Archer has accepted her destiny as Avenger, Saviour ... and Destroyer.

In public she is a sexy, sophisticated socialite; one of the beautiful people of Las Vegas. But she inhabits another world too: the city underworld that mortals can't see and could never enter ... a dangerous dimension where the agents of good and evil do battle.

But Vegas is still about winning big - or losing everything, and as the strength of the Light begins to fail, Joanna realises that to save her friends, her future, and both her worlds, she will have to gamble it all, embrace her dark side and confront the ultimate Shadow demon: her father.