The Invitation by Lucy Foley

The world's darkest hour had made her a ghost. Only he can see the light...
Rome, 1953: Hal and Stella meet by chance, two outcasts in a city far from home. Or perhaps it was the hand of the gods that night as the Eternal City welcomed the beautiful elite to its rooftops. It's a bright awakening for Hal, he believes he's found the love of his life - his Stella, his star.
To his dismay, the mysterious girl disappears as quickly as she swept in, and Hal's life as a journalist falls back into its familiar routine. But when he's asked by an Italian Contessa to assist on a trip of a lifetime - acting as the reporter on a tremendous yacht, skimming its way along the Italian Riviera towards Cannes film festival, the most famous movie stars and directors of the day as its cargo - Hal can't believe his good fortune. Especially when a familiar beauty becomes passenger.
Yet not all that glitters is gold, for many secret histories lurk beneath the surface of these Bright Young Things. Stella is bound to a powerful and profoundly dangerous man, and as Hal's obsession grows with every passing wave, they sail closer towards a tragic fate.