I'm not going to delve into the life and times of the Narnia's books' author, C.S. Lewis, or why he wrote these books or any other concomitant information. Instead, I'll just deal with Narnia itself, and the places and people contained therein.
I've read in various sources that this is not the order they're meant to be read in. My answer to that is that the order I've given is the chronological order the books take place in. The order in which the books were written is neither here nor there. To be strict, The Horse and his Boy takes place within The Lion, The Witch, and The Wardrobe while the four Pevensie children are still kings and queens of Narnia.
The seven books are a partial re-write of the New Testament. The Christian concepts of sin and redemption are all over these works. Homilies about messing with things beyond your understanding abound. There is the promise of an abolition of evil and a new era of goodness too.
I first read The Magician's Nephew as a kid of seven or eight. I was entranced by it. The connecting attics, Polly and the rings, the woods between worlds, Aslan the Lion, Uncle Andrew (who seemed a sinister Jon Pertwee to me), the doomed world of Charn, the coldness of Queen Jadis, Digory's sick mother...the world of Charn had a wonderful feeling of dying fall to it. The book is altogether a magical experience, full of wonder, revelation and delight. This is the book in which Narnia is created and an evil is unleashed into it, or a "Neevil" as the Talking Animals put it.
I turned up in some harsh doomed city on another plane
I couldn't believe the room I got or the guests I entertained.
The Church - Tantalized.
The Lion, The Witch, and The Wardrobe is the book most familiar to people. Four English children are exiled to their Uncle's mansion during the World War Two air-raids. Their Uncle being none other than Digory of the first book. While exploring this huge house, Lucy finds herself in the Land of Narnia after hiding in a wardrobe. Thoroughly engaging work with memorable characters such as Tumnus the Fawn, Maugrim the wolf, the White Witch, the Beavers, and of course, Aslan. The plot is twee; break the spell of eternal winter and let Santa Claus bring Christmas to Narnia. The four kids succeed at this and become kings and queens of Narnia, only to find themselves back home again after a hunting expedition in the Lantern Waste.
The Horse and his Boy is an interstitial work, for want of a better term. The events in this book take place while the four Pevensies are still on the thrones of Cair Paravel. I find this book to be the least of the septet. Why? It lacks the vivacity of the other stories and seems tacked on to me. That's not to say it's a bad book; not at all. Just not up to par of the others. Shasta is a boy living in the land of Calormen, which seems to be a land modelled after Ottoman-era Turkey. He meets a Talking Horse and the two set off for Archenland, a border nation between Narnia and Calormen. He meets Aravis, a princess-type, and they all flee to Archenland, to forestall a Calormene invasion.
Prince Caspian takes places centuries after the events of the first books. Telmarines, who seem to be the descendants of shipwrecked pirates, have assumed control of Narnia. The titular character is heir to the throne but his evil uncle Miraz has plans to the contrary (don't they all?). Anyhow, repression has taken over Narnia. Talking Animals do not officially exist, Aslan's name is forbidden and all that is good has fled and gone underground. The four Pevensie kids are swept into Narnia from a train station bench.
I like this book. The good guys have factions, which is odd for a children's book, and the badder of the two factions believe there is no place for humans (Sons of Adam and Daughters of Eve) in Narnia. So, effectively, we have a three-way conflict. This book introduces Reepicheep the mouse, Trumpkin, the Ford of Beruna, Aslan's Howe, Doctor Cornelius and others. The gooder of the two good guy factions prevail. Aslan sends the Telmarines home and Caspian assumes kingship of Narnia.
The Voyage of the Dawn Treader is different from the other six. It hardly takes place in Narnia at all. It's primarily a sea-voyage. For starters, the two older Pevensie kids, Peter and Susan, aren't in this book. We're introduced to their pathetic cousin Eustace, a self-centered brat if there ever was one. Lucy, Edmund and Eustace are sucked into a painting of a sailing ship and story begins...the now-grown up Prince Caspian is sailing eastward looking for seven friends of his father's.
The troupe have some extraordinary adventures; they encounter a despair-filled island shrouded in darkness, an island that has water that turns everything into gold, the island of the Dufflepuds, one-footed nonsensical dwarves, the island of Ramandu and finally the far land of Aslan himself. Eustace learns humility and the fate of the the seven friends is learned. Great stuff.
The Silver Chair sees Eustace returning, this time with a school-friend, Jill Pole. The Pevensie kids are absent altogether from this entry. Seventy years have transpired since the events in The Voyage of the Dawn Treader took place. The aged Prince Caspian's son Rilian has gone missing. The two schoolmates travel north across Narnia into the Ettinsmoor and meant Puddleglum. Puddleglum is a Marsh-wiggle, a tall and lank man-like being given to gloom and doom. A bit like A.A. Milne's Eeyore. With Puddleglum, who calls himself a "respectabiggle" while drunk, they go in search of Prince Rilian.
They meet the Lady of the Green Kirtle and the Giant's of the city Harfang. The giants have plans for the friends, not nice plans, and they make their escape from Harfang. They make their way into the Underland, a sunless realm that includes a sleeping Father Time, the land of Bism, and an enthralled nation of gnome-like people. The companions find the Prince and free him of the enchantment he was under, courtesy of the Lady of the Green Kirtle. The eponymous chair, it seems, held the power of thrall. Liberated, the Prince returns to Narnia.
The Lady isn't the White Witch, yet it would appear they shared common goals. The Lady is a far more subtle and charming person than the arrogant and cruel White Witch. How the Lady got into Narnia isn't recorded.
The Last Battle is analogous to the Book of Revelation in many ways. We have deception, apocalypse, the end of the world and a new era all wrapped up in one here. This book, more than any of the others, is full of allegory. Shift the ape is conning Narnia into thinking his friend, Puzzle the donkey, is Aslan, and that the evil god of Calormen, Tash, is Aslan by another name. This sets into motion terrible things; the Pevensie children (minus Susan, who has since abnegated Narnia for "real life" affairs) and Jill and Eustace are summoned by Aslan to right wrongs. The forces of evil, Tash, the Calormenes, the dwarves etc, are pitted against Peter and his friends. Read the book if you want to know how it all ends.