The first place to look is Chris Hill's The World Turned Upside Down, which is the best scholarly introduction to the Diggers.

Part of the problem when assessing the Diggers, is that they were a relatively short lived and small movement. Perhaps as many as twenty people assembled on St George's Hill to begin digging. They lasted only a few months, and despite their success in publishing, earned very few converts.

The other element to consider is that while the Diggers advocated for a radical egalitarianism, their attempt to resist the enclosure of the land was not unique to them. There are constant disputes in the English court system over the rights of the commons and the ability of the Lords to enclose the land. The land the Diggers were working on was essentially refuse land. Some of their writings, such as A Declaration from the Poor Oppressed People of England read like a social revolutionary's manifesto. While these writings are important, they are still saturated with the claims that God had inspired the "True Levellers," and that the Diggers claimed their liberties came from God, as exemplified in the Bible. The best way to read the Diggers is to read them as they saw themselves: a group of God's chosen people, inspired through a direct relationship with God, to challenge the social order as radically as the Parliamentarians had in their war against, and eventual execution of, King Charles.

The historiography of the diggers is covered in Bradstock, Winstanley and the Diggers 1649–1999. Another good reference is Woolrych, Britain in Revolution. Obviously the best place to look is in the writings and publications of the Diggers themselves, especially Gerrard Winstanley. I can't seem to find his writings for free online, but there are modern editions or his writings are on Early English Books Online, if you have a University subscription.