What are some themes in Ernst Theodor Wilhelm Hoffman's writing?

Hoffman was a German writer whose works spearheaded much of what we refer to as Dark Romanticism.
A principal theme in his writings is the issue of illusion versus reality—unsurprisingly so for his period. Madness is a frequent concern as well. His characters often exist in a surreal dimension in which we cannot know for certain how much is truth and how much simply a projection of a character's mind and violent emotions. In the story "The Sandman," the protagonist, Nathanael, is driven mad by visions that have haunted him since childhood of a man named Coppelius who is attempting to create an artificial human. Later, Nathanael falls in love with a girl named Olimpia, who turns out to be just such a being, an automaton. Nathanael kills himself, jumping to his death from a steeple.
It's not hard to see in this story the antecedent of the material from which Nathaniel Hawthorne and Edgar Allan Poe created their fiction. In "The Sandman," we also see a development paralleling that in Mary Shelley's Frankenstein, in which speculative science is merged with gothic horror. Throughout "The Sandman," we have no sure way of knowing how much of the action is being presented as "truth" and how much is merely a dreamlike extension of Nathanael's troubled soul.
The same is true in other works of Hoffman's, such as the short story "Don Juan." This work is a kind of philosophical interpretation of Mozart's opera (Don Giovanni in Italian) as well as a horror story. A man attending a performance of Mozart's opera communes with the opera's characters and becomes a part of their universe, while understanding that Mozart's music has created a higher reality than the actual reality of the material world. Hoffman's vision laid out in this novella has become a cultural trope regarding Mozart, the Don Juan story, and the power of music to create an independent reality. (Hoffman admired Mozart so much that he changed the "Wilhelm" of his middle name to "Amadeus".) A similar (though less intense) exalting of music occurs in his story "Ritter Gluck," in which the narrator meets and communes with the spirit of the composer Gluck.
Hoffman is also sometimes given credit for the invention of the detective story—and in this, he anticipated an achievement often attributed to Poe. The novella "Mademoiselle de Scuderi" is a work of historical fiction dealing with a series of robberies and murders taking place in Paris during the reign of Louis XIV. The themes of violence and madness are present here but in a less surreal form than in many other stories. There is also a proto-feminist element in the story given the title character's activity in uncovering the mysteries surrounding the crimes.
Hoffman's oeuvre as a whole can thus be seen as seminal for the Romantic period and influential in the realm of philosophical interpretations of the arts.