Greek tragedies come in threes with a satyr play to top them off. Prometheus Bound was the first of the Prometheia trilogy, and the only extant play in the trilogy. (The only extant trilogy by Aeschylus is the Oresteia, which will also eventually be a Fortnightly Book; there is no extant four-play set by Aeschylus, despite the fact that all the plays we have would have belonged to such a tetralogy.) We know the names of the other two tragedies in the trilogy: Prometheus Unbound and Prometheus Fire-bringer. We have fragments of the second, and know that it concerned the unbinding of Prometheus by Heracles. Of the third play in the trilogy we know practically nothing that is not speculative. We can reasonably guess that it would have reconciled Prometheus and Zeus and, possibly, provided an origin story for the torch relay the Athenians ran in honor of Prometheus.
Prometheus Bound is famously a play without much plot -- it would not be far off to say that almost the only thing that happens in it is that Prometheus is bound. Most of the actual story has already happened when the play opens (leading to an occasionally rebellious minority of scholars to suggest every so often that it may have actually been the second in the play, with Prometheus Fire-bringer actually being the first). It is a tragedy of character, then, rather than a tragedy of plot. Prometheus comes off sympathetically, and Zeus badly; the play ends with Prometheus accusing Zeus of injustice, with nothing forthcoming in response. But there is much foreshadowing of what is to come.
Shelley's Prometheus Unbound was intended to be a new sequel to Aeschylus's play, and yet not just a sequel. He wrote it while touring Italy with Mary Shelley -- reflecting on it in Milan, beginning it in Rome, and finishing it in Florence. If Prometheus Bound ends with the injustice of Zeus, Prometheus Unbound ends with the victory of Prometheus.
The Heritage Press volume also contains Mary Shelley's "Notes on Prometheus Unbound". Her explanation for Shelley's choice of subject:
The father of Greek tragedy does not possess the pathos of Sophocles, nor the variety and tenderness of Euripides; the interest on which he founds his dramas is often elevated above human vicissitudes into the mighty passions and throes of gods and demi-gods: such fascinated the abstract imagination of Shelley.
The volume is illustrated by John Farleigh, best known for his wood engravings, with sixteen full-page line-and-wash drawings. Aeschylus is translated by the classicist and novelist Rex Warner, who provides the introduction. It is typeset in Spectrum typeface by Hendrik Clewitts, a former assistant to Jan van Krimpen, who invented the font.
