Iroquois people, who originally came with the other Native Americans, first settled in the north-eastern part of North America around 1000 AD. They probably moved northward up the Susquehanna river (from modern Maryland) because of the global warm weather between 1000 and 1300 AD, just as the Inuit moved east and the Vikings moved west because of it. The Iroquois took their land from a smaller group of nomadic people we call the Woodland people. In the 1200s AD, for instance, the Cayugas (a kind of Iroquois) drove the Allegans away from the north end of Owasco Lake (now the town of Auburn), a trade town where two important trails crossed. The Iroquois didn't call themselves "Iroquois", which is an Algonquin insult meaning "snakes". They called themselves the "Haudenosaunee", meaning "people who live in longhouses." Or they called themselves by the kind of Iroquois they were - the Cayugas, the Mohawks, the Oneida, or the Seneca, for instance.
Around 1350 AD, the warm weather ended, and the environment began a "Little Ice Age", with colder weather. The Iroquois started to fight a lot of wars around this time, and they started to build their villages on high ground and surround them with strong log walls. One of their main enemies was the Algonquin, who were trying to move further south where the weather would be warmer.
At some point around the 1400s AD, the Iroquois formed a confederacy (con-FED-ur-ah-see), which is a sort of club or organization. This was an agreement between the different groups of Iroquois - the Mohawk, the Oneida, the Cayuga, the Seneca, and the Onandagua - to get along and fight as allies against their enemies, instead of fighting each other. This agreement was recorded using wampum.
The Iroquois: The Six Nations Confederacy, by Mary Englar (2006).
If You Lived With The Iroquois, by Ellen Levine (1999). Written - very lively and with a lot of good detail about daily life. I really liked it.
The Iroquois, by Barbara Graymont (2004). More detailed information.