In the book life of pi who is narrating the chapters that include italicized text?

They are the author's memories and impressions with the man he based Life of Pi off of. If you read the introduction (which is all in italics) he talks about how he came to write the book, and one thing the author mentions is the first meeting of real-life Piscine. So while the story is going on, the author adds in his thoughts on the main character.