But there's more to it than that. At the end of The Phantom of the Opera the Persian gives a rather brief narrative of Erik's life over a few paragraphs. Phantom fleshes out that narrative with more details about his time with the gypsies, his apprenticeship as an architect in Rome, etc. So, yes, we do know how Erik and the Persian met, but the details are thin in the original work, and Phantom fills in the holes rather well, in my opinion. It is certainly not a required read to appreciate The Phantom of the Opera, but it is a satisfying read for anyone who enjoys the story.