Chinese and Indian military strategists such as Sun-Tzu and Chanakya contain information on deception and espionage . Chandragupta Maurya, founder of the Maurya Empire in India, used assassinations, spies and secret agents. The ancient Egyptians had a thoroughly developed system for the acquisition of intelligence, and the Hebrews used spies as well, as in the story of Rahab. Spies were also prevalent in the Greek and Roman empires. During the 13th and 14th centuries. Feudal Japan often used ninja to gather intelligence. Many modern espionage methods were well established even then. Aztecs used Pochtecas, people in charge of commerce, as spies and diplomats, and had diplomatic immunity. Along with the pochteca, before a battle or war, secret agents, quimitchin, were sent to spy amongst enemies usually wearing the local costume and speaking the local language, techniques similar to modern secret agents.

secret spies living next door? 

You see them every day, you wave to them as you drive to work. You think you know your neighbors, but do you? How do we know if Jerry down the street is a teacher, or Marry who has lived behind you for years is actually a manager at drug store. Easily people can live double lives maybe your close friends work for government. Hollywood has filled our brains with people who they think are spy agents from 007 to Jason Born. How do we know what real spies look like? The answer is we don't, so next time you see your neighbor, think do you really know them as well as you think.  
During WW2 agents were armed with cool gadgets. We all have seen spy movies where they have very advanced technology. We have changed much in the technology department. In 40s a camera in a button on a coat with a huge shutter in the pocket, was pretty advanced compared to what a average camera looked like. A simple domino can hold a secret message behind its dots. An umbrella can projectile a sharp knife into a unsuspecting terrorist. Now a days we can use a camera size if a kernel. Write seemingly unbreakable codes into a grain of rice. A gun the size of a banana can shoot like a machine gun. Our technology is constantly changing and improving. Maybe in he future we will be using phones that are see through, or maybe drive a car that will run on corn and egg shells.
If you have ever read a good spy novel? Then, you know what its like to sit at your edge of your seat reading every word with pure excitement, dying to turn the page and know, what's next? Authors of these said novels work vigorously to produce a one of a kind novel that will captivate their audience with spy adventure. Most of these spy novels are fiction. Unfortunately not all spies are involved in covert deep cover affairs. Spy novels mix fact and fiction to keep readers reading.
This interview gives a great look at the book double cross. All the questions are answered in this interesting interview. Why agents became double agents. How Britain double crossed Germany. 
All of these agents began working for German government but soon all of these agent double crossed Germany to become British agents. Why would someone became a double agent? Some would do it for personal gain, or because they don't enjoy watching the country who they sworn to protect hurting innocent lives, or the agent tries to seek approve from multiple sources because as a child they weren't given acceptance from their parents,  or maybe they are bored.  A double agent is a person who engages in activity for two intelligence or security services. The double agent often has access through his travels for the other service to positive intelligence on that country, or on third countries of interest to the controlling service. A double agent can be an asset to a country or there worse nightmare.