★★★★★ The Lion, the Witch and the Wardrobe, by CS Lewis
The Ur-portal fantasy novel
The Lion, the Witch and the Wardrobe
CS Lewis
The Ur-portal fantasy novel
I was five years old the first time I read C.S. Lewis's The Lion, the Witch and the Wardrobe. My first experience of it was in audio format -- my mother read it to my sister and me. My sister remembers that I told them, "This is about Jesus. Aslan is Jesus." They were skeptical, but they eventually realized I was right. Given that all three of us loved it, I think this shows that you can appreciate LWW regardless of your feelings about the religious message. (In this it is distinct from Lewis's Christian fantasies for adults, e.g. the Space Trilogy, which I found unbearably heavy-handed.)
I later went on to own and read (by myself, in ink on paper format) the entire Chronicles of Narnia. I can remember being distinctly annoyed at the illustrator for LWW, who, despite the clear statement "it was all three-legged stools in the Beavers’ house except for Mrs. Beaver’s own special rocking chair beside the fire" drew Mrs Beaver sitting in an ordinary four-legged chair.
These seven novels are, of course, classic speculative fiction for children, and LWW is one of the best of the seven. For the record, I think I would choose The Silver Chair as the best and The Horse and His Boy as the least impressive. But you should just read them all -- they're short enough.
I doubt that the Chronicles of Narnia were the first portal fiction, but by now I feel confident in saying that they are the earliest example to survive and still be widely read in 2023.