The Corpus Hermeticum is not a book but a gathering of seventeen or eighteen short Greek treatises, most in dialogue form, assembled from separate sources and not by a single hand. They do not agree with each other. Some are broadly optimistic about matter and the cosmos, some are sharply dualist and treat the body as a prison, and the collection has been read as evidence for a school and as evidence for a milieu, depending on which treatises a reader weights.
The first and best known is Poimandres, in which a being of that name appears to the narrator and delivers an account of creation, the descent of a divine humanity into nature, and the soul's return upward through the planetary spheres, shedding at each one the quality that sphere imposed. That ascent through the spheres is the single most influential image in the collection and it recurs across late antique religious literature.
What made the corpus matter to Europe was a translation. Marsilio Ficino was working on Plato for Cosimo de' Medici when a Greek manuscript of the Hermetica reached Florence, and Cosimo had him set Plato aside and do the Hermetica first. The Latin version, printed in 1471, went through dozens of editions and put these texts at the centre of Renaissance thought for a century and a half, until the dating collapsed.