Harbinger Wars doesn't get the accolades it deserves. One of the only cross-overs, from any company, that I've actually enjoyed.....After you read the Harbinger Wars deluxe hardcover, try to make sure to get Bloodshot volume 4: H.A.R.D.Corps, which continues the storyline a little bit. Also make sure to get Imperium, which starts up after Harbinger ends. Imperium focuses on Harada, which is a character you'll want more of after you read Harbinger Wars.
Also, get the Rai deluxe hardcover! It collects issues 1-12, after that it's the 4001 AD mini/event (Along with Rai continuing with issue #13, which starts up a storyline showing past Rais).....Continuity-wise, you don't need to read anything before getting into Rai.
Aside from the earth constantly getting damaged by Father, Gilad's there to rescue the Geomancer.