The Magician's Nephew by C.S. Lewis

One of my favorite books in the Narnian series is The Magician's Nephew by C.S. Lewis, first published in 1955. Not the first book written, but the first chronologically, which tells the story of friends Diggory Kirk and Polly Plummer. The story starts in the late 1800s (when Sherlock Holmes was still living on Baker Street) and the two friends meet when Diggory and his ill mother move in with her brother and sister, Uncle Andrew and Aunt Letty, an old bachelor and old maid, living in London.

Diggory's father is away in India, and isn't able to be in England to help his family. Diggory and Polly, in their explorations of their row of houses, accidentally end up in Uncle Andrew's private study. Without their consent, he sends them off on an adventure into other worlds where they end up meeting Jadis, the evil queen in a place called Charn, and accidentally bring her back to London. In their attempts to get her out of our world, they accidentally take her to Narnia, a new world just beginning. But fotunately, Aslan the great lion and creator of Narnia, has a plan to deal with Jadis.