The Chronicles of Narnia : book one (or six, depending on whether you go by Narnian history or publication date)
In the summer of 1900 Digory and Polly meet. Digory has come to London because his mother is very ill and he isn’t happy about it at all. While playing in attics of their terraced houses they come through the wrong door and end up encountering Digory’s uncle. Uncle Andrew is not a very nice person. He has been experimenting on hamsters, but the children will make much better subjects, they can do much more than a rodent ever could. So Andrew tricks Polly into picking up one of the magical rings he has created and sends her off into another world. Digory will further test his magic when he goes after her and brings her back.
And so the story of the very creation of Narnia begins.
It is also the story that explains how that infamous iron lamp-post found it’s way into a wood in Narnia.
I first read the Narnia books when I was a child, I’m pretty sure I read them in publication order, which means The Lion, the Witch and the Wardrobe first, and if you haven’t read any of the books that is the way I would recommend starting the series. For a reread, in-story chronology if fine. This read was a reread. Probably a reread of a reread if I’m honest. As a child I read and reread books over and over again, much like the way dvds now get worn-out through rewatching.
The Magician’s Nephew is an easy read, more light-hearted in many ways that the other books in the series, and not really about events in Narnia but about how Narnia came to be. And how the White Witch found her way into Aslan’s creation.
It also explains that lamp-post, and even how that wardrobe lead to Narnia.
Of course reading it as an adult means you notice the Christian origins much more than as a child. Back then all that went a bit over my head. I enjoyed the books because they were great fun rather than for any religious aspect, I never noticed the religion until someone mentioned to me as a teenager. And that’s how they should be read if you ask me. And this one starts a bit slow, but eventually is has that adventure, plus horses and talking animals and flying horses and evil witches.
It isn’t the best in the series, but I enjoyed it and continue with my reread of the series.