What kind of people became spies? Who did they work for? What methods did they use? Were there different kinds? Was it something that existed professionally the way it does now, or something only on an ad-hoc basis? (Ie could you "be a spy" as a first or second job, or could we only say that someone "was spying" on someone else?)
I wrote about spying and spies in the high medieval Islamic world for medievalists.net a little while back, if you're interested.
An excerpt:
If we can believe the chroniclers, dynasties and empires from the Rashidun caliphate to the heydey of the Turks secured their internal and external power through a basic principle: “the ruler ought to plant spies on his subjects and associates; eyes that spy on their whereabouts, their news, especially if there is any doubt about them.” Spies were so zealous and persistent that the Seljuq Turks had to regulate boundaries for their own agents (don’t unveil women or break down locked doors), and letters of advice warn anxiously not to act against an ally on the word of a spy until you are really, really sure. By the end of the seventh century, the expectation of espionage was so ubiquitous in Basra that groups of people would simply turn away any unknown face.
With both suspicion of and need for secret agents so high, Muslim rulers found themselves turning to the ultimate undercover agents: women.
The important thing to bear in mind regarding spying in that era is that without the ability to communicate information in real-time, spying didn't play the role in warfare/conflict that it does today. Spying was more about gathering information about commerce, geography, local politics etc. However, rarely would this information have a decisive impact on battles and the like, because by the time it reached its intended audience, much would have changed.
The Romans were some of the first to structurally incorporate intelligence into their armed forces, with speculatores and exploratores (scouts) often being sent ahead to glean information as to terrain, enemy presence, etc. Caesar's campaign to pacify Gaul also used indigenous peoples as informants to discover how each tribe operated and their behaviours, in order to exploit inter-tribal divisions and prohibit a strong anti-roman federation appearing. The Roman intelligence network benefited from the infrastructural advances of the Roman Empire - particularly its roads - which allowed information to be carried relatively quickly.
By the time mid-Medieval Europe rolled around, most of the Roman infrastructure was degraded, making it difficult to move armies around at any great speed. The flow of information was slowed, and there was no great imperial expansion to warrant the use of intelligence in such a manner. During the Teutonic Crusades into the Baltic region in the 13th century, the tactic was to build forts along the coast, from which units could launch exploratory missions to gather information about the surrounding area.
In terms of inter-state spying, in the time period you are asking about, spying wasn't much of a full time thing, and it definitely wasn't a profession in the way it is today. Your nation's spies may be merchants, traders, or heralds; those who would be working in crossroads of people and information. They would send information back through trusted couriers, but it would often take weeks to reach its intended audience.
Source: Intelligence in War, John Keegan