From Eternity to Here — Michael Sellers

Lady Alexandra Selve was twenty, shortly to inherit the vast Selve fortune and about to hand it intact to a bizarre religious sect: the Cosmotheistic Birds of Passage.
Her mother was naturally determined to prevent her daughter giving away her inheritance. She'd failed to get any result from her approaches to the Home Office, the police and the Archbishop so, as a final desperate resort, she hired a private detective, Calouste Fisher, to persuade her daughter to abandon her commitment to this dubious 'charity'.
While Cal was determined to fulfil his commission (and earn a fat fee for his trouble), the guardians of the Birds of Passage were equally determined that no one should reach Lady Alex and remove her from their influence. And as Cal becomes more heavily embroiled in the curious life of the sect, he realises they have something more sinister to protect than the fortune of one little rich girl.