About the book (from Wikipedia):
The Periplus of the Erythraean Sea or Periplus of the Red Sea is a Greco-Roman periplus, written in Greek, describing navigation and trading opportunities from Roman Egyptian ports like Berenice along the coast of the Red Sea, and others along Northeast Africa and the Sindh and South western India. The text has been ascribed to different dates between the 1st and 3rd centuries CE, but a mid-1st-century date is now the most commonly accepted. While the author is unknown, it is clearly a firsthand description by someone familiar with the area and is nearly unique in providing accurate insights into what the ancient European world knew about the lands around the Indian Ocean.
The Erythraean Sea, or in modern spelling, Eritrean Sea is presently a Greek name for the Red Sea; to the ancient Greeks it included the Indian Ocean and the Persian Gulf.