The first half of this novella can be counted among the most remarkable writing I have ever read. its sense of unstated terror and crisp, nightmarish atmosphere; its portentous introduction and proliferation of the double theme; its destabilising of its own narrative, where the violence of the language and the force of the metaphors makes the abstract material, and the material abstract; its evocation of London as a menacing organism, a miasma-wheezing labyrinth, with an economy that defeated Dickens, with streets and buildings embodying human flaws; its characterisation of a grim, barren, self-destructive men's world - all this take the novel away from the generic sensationalism or pseudo-scientific philosophy of the horror genre towards the metaphysical anxieties of Chesterton and Borges.
The rest is more familiar, made complex by innovative structure, ambiguous narration and a startling use of imagery. this is not a simple tale of man's good and evil side; in its admission of an ungraspable, shifting, multifarious existence, shown here in character, place and language, where metamorphosis is the only rule, we can see why Nabokov considers Stevenson a master. And yet the book also works as a lean, compelling thriller, even if, like everyone, you already know the twist. Emma Letley's introduction and notes are over a decade old, and need updating.