First, a correction. The story of Byblis and Caunus was known prior to Ovid. A discussion of the tale is found in Parthenius of Nicaea's Erotica Pathemata (Of the Sorrows of Love), and it points to a few earlier sources where the story can be found in one form or another.
Parthenius was brought to Rome in 72 BC, and is believed to have tutored Virgil. So while he apparently died in 14 AD, his work was probably produced much earlier.
Thanks to Theoi, the Erotica can be found here. What's striking is how devoid of literary elaboration these tales are in Parthenius' dry hands. Here, for example, is his story of Byblis and Caunus:
There are various forms of the story about Caunus and Byblis, the children of Miletus. Nicaenetus41 says that Caunus fell in love with his sister, and, being unable to rid himself of his passion, left his home and traveled far from his native land: he there founded a city to be inhabited by the scattered Ionian people. Nicaenetus speaks of him thus in his epic: –
Further he42 fared and there the Oecusian town founded, and took to wife Tragasia, Celaeneus’ daughter, who twain children bare: first Caunus, lover of right and law, and then fair Byblis, whom men likened to the tall junipers. Caunus was smitten, all against his will, with love for Byblis; straightway he left his home, and fled beyond Dia: Cyprus did he shun, the land of snakes, and wooded Capros too, and Caria’s holy streams: and then, his goal once reached, the built a township, first of all the Ionians. But his sister far away, poor Byblis, to an owl divinely changed still sat without Miletus’ gates, and wailed for Caunus to return, which might not be.
However, most authors say that Byblis fell in love with Caunus, and made proposals to him, begging him not to stand by and see the sight of her utter misery. He was horrified at what she said, and crossed over to the country then inhabited by the Leleges, where the spring Echeneïs rises, and there founded the city called Caunus after himself. She, as her passion did not abate, and also because she blamed herself for Caunus’ exile, tied the fillets of her head-dress43 to an oak, and so made a noose for her neck. The following are my own lines on the subject: –
She, when she knew her brother’s cruel heart, plained louder than the nightingales in the groves who weep for ever the Sithonian44 lad; then to a rough oak tied her snood, and made a strangling noose, and laid therein her neck: for her Milesian virgins rent their robes.
Some also say that from her tears sprang a stream called after her name, Byblis.Ovid clearly went to town on this story, reversing the "poles" so that Byblis is the afflicted lover, introducing all the paraphenalia of writing, developing the passion and her means of relating it to her brother through several phases involving elaborate arguments, duplicities, and reversals.
So a question we might want to ask as we explore Byblis: Why does Ovid use this tale to give us, as Anderson notes, the first detailed description of a writer at work of which we have any record? Is there a reason why the art and craft of writing enter the Metamorphoses precisely at this point?
Note that the "Sithonian lad" of Parthenius is poor Itys, whom we last saw in the tale of Procne, Philomela, and Tereus in Book 6.