In addition to the websites suggested in an earlier post, you might want to check out some of Rolf Slotboom's columns. PLO is one of his main games. He writes for CardPlayer and an archive of all CardPlayer columns over the past few years are available at
www.cardplayer.com
Slotboom also has a website that might have more PLO info. I don't have the address handy but I bet Google can find it.
Bob Ciaffone also writes for CardPlayer and some of his past articles probably deal with PLO.
I don't think you've missed any major PLO books. I think there's a chapter on it in the new SuperSystem, but I haven't seen it.