The Chronicles of Narnia (1950-1956) by C.S. Lewis
Including: The Lion, the Witch and the Wardrobe, Prince Caspian, The Voyage of the Dawn Treader
The defining portal fantasy series. Much like epic fantasy after The Lord of the Rings, it’s pretty much impossible to talk about any book involving a magic door to another world without bringing Lewis into the conversation. The trick, though, is that the sense of wonder you felt the first time you read The Lion, The Witch, and the Wardrobe is still there. Aslan-as-Jesus-figure is a well-worn genre joke at this point, but Lewis’s writing has such a transportive quality that the all-knowing lion is still effective and affecting. And that’s really the key to any lasting portal fantasy -- the book was the magic door all along.
The Hitchhiker’s Guide to the Galaxy Series (1979-2009) by Douglas Adams
Including: The Hitchhiker's Guide to the Galaxy, The Restaurant at the End of the Universe, Life, the Universe and Everything
Adams is easily one of the funniest writers who’s ever lived -- The Hitchhiker’s Guide to the Galaxy has helped shape the sense of humor of several generations at this point, and shows no signs of ceasing to do so anytime soon. Notably, Hitchhiker’s Guide was a multimedia sensation in an era before that was a common marketing strategy, originating as a successful radio drama before being turned into an indelible series of novels, a television show, a stage production, a video game, and, decades later, a decidedly mediocre film. (There is, of course, another TV adaptation in the works.) It’s remarkable that these books were an adaptation of another medium, because they feel so thoroughly, zanily lived-in -- an eternal testament to Adams’ talent.