The story of the Egyptian gods Isis and Osiris, their brother Set, their child Horus, and the sun god Ra is well known from a mixture of native Egyptian texts and from texts written by the 2nd century AD Greek biographer and philosopher Plutarch, and the slightly later Roman novelist Apuleius.
The Egyptian texts have been conveniently collected by reshafim. Scroll down on this page from the reshafim site for texts on mythology.
Plutarch's essay on Isis and Osiris omits many of the more salacious details to be found in the Egyptian texts but starts off with a more straightforward narration of the story before plunging into Plutarch's interpretations of the story as an allegory. Apuleius finished off his novel The Golden Ass with an account of how the main character Lucius was restored to human form by the goddess Isis and was initiated into the mysteries of Isis and of Osiris. An Elizabethan translation can be found here and a more modern, but less legible, one here.
And to finish off with, here is "Isis and Osiris" from Mozart's "The Magic Flute":