When considering classic tales of Halloween, the question to ask is, "Where is the monster?"
In Bram Stoker's Dracula, the monster is the myth / legend that everyone warns you about but you ignore ... until it's too late.
In Mary Shelley's Frankenstein, the monster is your view of how the world should work, put to the test in an experiment gone horribly, horribly wrong.
In Robert Louis Stevenson's Dr. Jekyll and Mr. Hyde, the monster ... is you.
Thanks to popular culture, this is one of those stories that people think they know without actually reading the book. And that's a shame, because this is a novella of suspense where the narrative can surprise you - provided you suspend your knowledge of the plot as provided by pop culture.
Since this is in public domain, there shouldn't be a problem finding a copy. However, if you'd like to consider a recommendation, the Penguin Classics version has some useful footnotes.