Mary: or, The Birth of Frankenstein by Anne Eekhout

As darkness falls and storms rage over Lake Geneva, a group of friends gather in a candle-lit-villa.
Among them are eighteen-year-old Mary and her mercurial lover Percy Shelley. As laudanum stirs their feverish imaginations, their host Lord Byron challenges everyone to write a ghost story.
Suddenly Mary is transported back to a long, strange summer in the wilds of Scotland, where she fell in love with the enigmatic Isabella Baxter.
As she remembers, something fierce and terrifying awakes within her. Now she will unleash it into the world.
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'A fantastically moody, unsettling novel, with a teasing, enigmatic atmosphere entirely its own' --Sarah Waters
'A bold new framing for questions about where we draw lines: between queerness and heterosexuality, the natural and the unnatural, and the imaginary and the real...' New Yorker