Alternatively, you could read the novelization. It's cheaper than getting two or more comic trades, and you get the relevant backstory as you go along.
God bless you.
Seriously, the prequel tie-ins are all enjoyable, but I would say that THE OMAC PROJECT and SACRIFICE are the most relevant to the plot of IC.
Regarding DAY OF JUDGMENT, all you really need to know is that the Spectre has gone berserk and the Shadowpact has formed to stop him. VILLAINS UNITED is great fun, and maybe my favorite of the bunch, but it's kind of a sideshow. For RANN-THANAGER, all that matters is that a bunch of heroes are stuck in the middle of a space war on the other side of the galaxy when all hell breaks loose . . . .
And, oh yeah, Donna Troy is alive again.