Spy novels are a particular kind of long-form fiction dealing with spies and their adventures. According to experts, the genre has been around in some form or another since the 1920s. Spy novels can be serious, or they may exist just for entertainment value, and some feature dashing heroes, while others have dark antiheroes that may be very hard to sympathize with. Most spy stories rely heavily on tension, along with heavy doses of action, and they also typically have extremely complicated plotting.
Many spy stories are known for being relatively fun for people to read. These novels often romanticize the experience of living as a spy, visiting exotic foreign locations and living with fewer rules than other people. The spy, usually a dashing hero, may face many dangers during his adventure and use all sorts of advanced technology to get out of jams.
Some other spy novels have a darker bent. These stories typically try to present a more realistic look at the world of international espionage. The spy may live in poverty, constantly afraid for his or her life and trying to avoid discovery by enemy forces. In many cases, these novels exist partly as a purposeful counterpoint to the more easy-going novels that generally exist purely for entertainment.
There are also spy novels where the main character may not even be classified as a "good guy" at all. For example, some spy stories deal with characters that are basically official government assassins. The stories may deal with the ethical quandaries these characters face in their day-to-day lives as they struggle with their own personal moral code versus their sense of duty to their government.
It's very common for spy novels to have very twisted and sometimes difficult plotting. For example, it is almost traditional for the typical spy novel to have unexpected betrayals and elaborate tricks. In many cases, characters are constantly plotting against each other in inventive ways that can be difficult to predict. There are even some spy novels that are known for being especially difficult for people to understand.
Partly because of the overall excitement and fast pacing associated with the genre, the spy novel has often been translated into movie form. There are many spy films that are directly based on famous books. Some of them have been box-office blockbusters with multiple sequels, and in some cases, the films have become more famous than the novels they were based on, at least among those in the general public.