This group of texts concerns a form of fantasy that would have been defined by J. R. R. Tolkien as a "Primary World" and is called by more recent critics a "portal-quest". Such worlds are initially set in a realist literary place that is strongly identifiable in time and space. However, this space only exists in order to be contrasted with another fantastic one to which the characters are going to travel, and from which they will return.
The earliest and most influential example of this is Thomas More's Utopia which uses the difference between the two worlds to question and critique its own time. In a more extreme way Jonathan Swift does the same thing in Gulliver's Travels – using the differences to satirise his own society and its values. Twentieth-century children's fiction also frequently has this kind of "bridge" text which starts and returns "on earth", probably because of the need to reassuringly return safely home at the end of all adventures. However, time does not always correspond to that on Earth – as in The Lion, The Witch and The Wardrobe when the children become full adults in Narnia but return to earth as children just after the moment when they left (very unsettling!)
Thomas More, Utopia
Jonathan Swift, Gulliver's Travels
Richard Jefferies, After London
C. S. Lewis, The Magician's Nephew
Philip Pullman, The Subtle Knife