There were four lead-in mini-series and then a million tie-ins in the regular series. The four minis were OMAC Project (very important pretty good), Day of Judgement (somewhat important entertaining), Rann-Thanagar War (completely unimportant but alright) and Villains United (the actual story has little to do with IC but it's damn entertaining). The most important tie-in trade to get would be Superman Sacrifice but those events are covered in OMAC indirectly. The IC Companion trade collects the "specials" that wrap up the 4 mini-series but they're not really necessary. The Superboy/Luthor story happened in Teen Titans / Outsiders: The Insiders but reading that isn't terribly important to IC. Identity Crisis was the initiation of the larger storyline but it's only indirectly tied to the actual plot.