Harry Potter
The Harry Potter series of books by J K Rowling are an interesting example of the use of the imagination by a writer.

Is there such a thing as the "magic" she describes? Clearly not. It is entirely the work of her imagination (and that of the reader, who can pretend to believe in it, while reading the books - "suspension of disbelief").

But there are other themes in the books. For example, she writes about a school called Hogwarts. It is very selective and the students live at the school. Only a small proportion of people can go there - they have to be born with the right qualities. Are there really such schools in England? Of course there are. Their names are Eton, Harrow, Rugby and Winchester (and many others, less famous). These are expensive boarding schools (Public Schools - in reality Private, not run by the State) where the upper classes send their children. In the past it was the graduates of these schools who ruled the British Empire, and Britain itself. To some extent they still do.
There is a long tradition of novels about life in such schools. The first of this genre was probably Tom Brown's Schooldays by Thomas Hughes about life at Rugby School in the time of Dr Arnold, a famous reforming headmaster. A later, rather satirical series, featured Billy Bunter at Greyfriars school. Rowling has taken elements of those school stories and incorporated them into her series about the imaginary Hogwarts.

Many of her 'magicians' despise ordinary people (Muggles), just as many of the upper classes of the past despised ordinary people (oiks), who in those days did not even have the vote. So Rowling's novels are not as imaginary as we might at first think.

In the past ordinary education was available only to a small part of the population - much as education is in Africa today. Those who had education had great power over those who didn't get to school. Rowling's "wizards" are a bit like that minority.

There are other themes in Rowling's books. The fourth book has a plot very similar to a spy novel by Len Deighton - The Ipcress File. In Deighton's book the main theme is that no-one can be trusted in the world of spies as people may be working for 'the other side' and it is very difficult to know what other people are thinking, or to whom they are loyal. Rowling's characters are ambiguous in exactly this way, and at the end of the book various characters who seemed to be of the 'good' party turn out to be of the 'bad' party; and vice versa.
The theme running through all the books so far is a war of 'good' versus 'evil' - one of the main themes of all literature. In spy novels of the recent past it was about the Cold War between the western countries and the Soviet Union and its allies (1948-1990). Earlier, it was the war between the western countries (Britain, France and the United States) against regimes like those of Hitler (Germany), Mussolini (Italy) and Japan.