Indigo

British Fantasy Society: Best Novel winner (2000).

It is a color the human eye cannot truly see, a slice of the spectrum imbued with the promise of invisibility. But for Jack Chambers, the son of a scientist renowned as both a genius and a madman, it will lead to places of unknown treachery. As executor of his estranged-father's will, Jack is appointed two ominous tasks: publish Timothy Chambers' bizarre manuscript Invisibility: A Manual of Light, and track down an unknown woman who stands to inherit the substantial estate. Jack's mission leads him to reunite with his half-sister, Louise, now grown into a stunning woman. Bound by a tense attraction, Jack and Louise head to Rome, where a cultlike group pursues the intoxicating secrets of the elusive indigo – and where Jack perceives its horrid danger only when it's too late.