American atlas Book

This is the extraordinary story of Byron, the Shelleys, Percy Bysshe and Mary, Clare Clairmont, Mary's half-sister and Byron's doctor Polidori. Apart from Polidori's letters to his sister Florence, which Marlowe invented, all quotations and much of the dialogue are taken from the letters and journals of the people involved, or from contemporary sources (contemporary being 1969, when this book was first published by Jonathan Cape).

Clare falls pregnant by Byron but leaves Italy before the birth of the daughter who, although christened Alba, became known as Allegra; Shelley drowns in the lake and Byron, when not writing poetry, getting to know the Shelleys or impregnating Clare, undermines his doctor horribly. But the point of the story for me is that one night (at the end of Chapter 6, page 100 of my Penguin edition) this happens:

That evening, the thunder returned to Lake Leman. All five members of the circle gathered once again in the darkened drawing-room, but this time the subject of conversation was far different from any other so far discussed. A book had been taken from the shelves and been read merely to pass the time. It was called Fantasmagoriana, ou Recueil d'Histoires d'apparitions, de spectres, revenans etc., translated into French from the original German, and the moment it was read all past quarrels, squabbles, philosophies, polemics were put aside as a new theme took over their thoughts. It was a subject that had fascinated others for centuries and which now, in this ill-lit room battered by rain and storm, was to enthral the occupants for days, even weeks, and was to make one of them instantaneously and surprisingly famous. It was to cause nightmares and hallucinations and much screaming in the night, and yet was to produce one of the most original novels of the century. The subject, of course, was Ghosts.

And the book that emerged, of course, was Mary Shelley's Frankenstein.