As I write this in the summer of 2009, it's almost twenty five years since Crisis... was first published and there are already numerous sites dealing with both the story and its repercussions out in the web. Chief among them is, I think, Jonathan Woodward's excellent annotations site but there's also the ubiquitous Wikipedia page among others.
This is my take, done in the same style as my previous annotations which can be found in the Links page.
Issue 1 - The Summoning!
Issue 3 - Oblivion Upon Us