Start with The Lion, the Witch and the Wardrobe. If your copy is a modern HarperCollins edition, the spine will say book two and tell you to begin with The Magician's Nephew. Ignore it. Publication order is the better reading order, and the numbering on your books is a publishing decision from 1994, not an authorial one.
The reason is concrete rather than sentimental. The Magician's Nephew is an origin story: it explains where the wardrobe came from, why a lamppost stands in a snowy wood, and who the White Witch is. Every one of those is a payoff. Read first, it is a mildly odd children's book about two children and a magic ring. Read sixth, it detonates a series of small explosions in your memory of the books before it.
The two orders
Publication order: The Lion, the Witch and the Wardrobe, Prince Caspian, The Voyage of the Dawn Treader, The Silver Chair, The Horse and His Boy, The Magician's Nephew, The Last Battle.
Chronological order: The Magician's Nephew, The Lion, the Witch and the Wardrobe, The Horse and His Boy, Prince Caspian, The Voyage of the Dawn Treader, The Silver Chair, The Last Battle.
Note that the last book is seventh either way, and that the two orders differ only in where two books sit.
The controversy, honestly
The chronological case rests on a letter Lewis wrote in 1957 to an American boy, Laurence Krieg, whose mother thought the books should be read as published. Lewis replied that he thought the boy's order — chronological — was probably better, and that he had not planned the sequence in advance anyway. HarperCollins renumbered the books on that basis in 1994, and every edition since carries the chronological numbering.
The counter-case is that Lewis was being courteous to a child who had already read them, and that a remark in one letter is thin ground for overturning the order in which the books were written, published, and read by everyone for forty years. His stepson Douglas Gresham supports chronological; a large number of critics and readers, including the Narnia scholar Peter Schakel, argue for publication.
Recommendation: publication order, and not tentatively. It is the only order in which the reveals function as reveals. But this is a genuine disagreement with the author's own words on the other side, so anyone telling you it is settled is overstating.
The one place both orders are awkward is The Horse and His Boy, which takes place during the reign described at the end of The Lion, the Witch and the Wardrobe. Publication order puts it fifth, which is fine — it reads as a story told from inside a golden age you already know about.
Honest warnings
The Last Battle is a genuinely bleak book, and two things in it are seriously contested. The fate of Susan, dismissed for an interest in lipstick and invitations, has drawn sustained criticism from writers including Philip Pullman and J. K. Rowling. The depiction of the Calormenes, a dark-skinned southern people in turbans, is widely read as drawing on period stereotypes. Lewis's defenders have answers to both. The arguments are real, and worth knowing about before you hand the book to a child.
The Silver Chair and The Horse and His Boy are the ones adults most often underrate and children most often love.
Reading around Narnia
Lewis's own Surprised by Joy is his account of the imaginative appetite the books came out of. The Discarded Image, his last academic work, describes the medieval cosmos that Narnia quietly runs on, and Michael Ward's Planet Narnia argues that each of the seven books is built around one of the seven medieval planets — an ingenious thesis that is not universally accepted but is fascinating either way. Alister McGrath's C. S. Lewis: A Life and Alan Jacobs' The Narnian are the two standard biographies, with Jacobs the more readable.
For a very different sort of portal fantasy afterwards, see The Chronicles of Amber.
Follow the full ordered path here: Narnia Reading Order: Publication or Chronological?.
