This is the sixth book published in C. S. Lewis's very famous The Chronicles of Narnia, but chronologically the first in the series as it occurs 1000 years before the first published book, The Lion, The Witch, and the Wardrobe.
Wikipedia on the subject of ordering: "... publication order reflects Lewis's strategy for drawing readers into the world of Narnia. In the book he wrote first, Lucy Pevensie's discovery of the wardrobe that opens onto a forest and a mysterious lamp post creates a sense of suspense about an unknown land she is discovering for the first time. This would be anticlimactic if the reader has already been introduced to Narnia in The Magician's Nephew and already knows the origins of Narnia, the wardrobe, and the lamp post." I'm very much inclined to agree with this assessment ...
Digory (who is the "magician's nephew" of the title) is a young boy who goes exploring with his neighbour Polly. They encounter his Uncle Andrew, who uses them as guinea pigs in an experiment that sends them to another world - and from there, a couple more worlds. Digory unleashes evil, and manages to bring it to the very Christian creation of Narnia ... where humans take the lead over talking animals even though the animals are of the world and the humans aren't.
Uncle Andrew seems to be Lewis's condemnation of the limits of science: he's telling us "asking questions is good, up until it's not." And he makes it clear that questions of faith are where the questioning and research of science should end - basically "don't mess with things you don't understand" (which I thought was kind of the point of science ...).
I read a fair number of children's fantasy books. Some are thoroughly engaging, although the morality is often very black-and-white. But you can hope for, and often get, some tension. This one is just goofy. It's poorly structured, with the children spending the first third of the book in "the wood between the worlds" and on "Charn," which are unrelated to Narnia (except that they're all connected by "the wood between the worlds"). And then another third of the book is spent on the creation of Narnia, so we have about one third of the book left for another mini-plot, finally actually set in Narnia.
I remember the central five books as being good (although blatantly Christian) from when I read them as a kid - this one doesn't live up to that memory. But I've started re-reading "The Lion, The Witch, and the Wardrobe" ... I may have that memory blown out of the water, we'll see.