Dean Snow's The Iroquois is a good place to start. You might also be interested in Barbara Mann's Iroquoian Women. Snow's tackling the topic from the perspective of an archaeologist; Mann's coming out at it from perspective of a historian (and an Iroquoian historian at that). They don't always agree, but that's what makes reading them together interesting.

Lewis Henry Morgan's League of the Ho-dé-no-sau-nee Or Iroquois is "the classic" on the subject, but you'll probably want to read at least one of the other two.

A few other books worth checking out:

• Iroquois Diplomacy on the Early American Frontier by Timothy Shannon
• The Ambiguous Iroquois Empire by Francis Jennings
• The Ordeal of the Longhouse by Daniel Richter