Someone needs to add this to the FAQ:

Books:
Omaha Poker Holdem by Bob Ciaffone (Note: This is out of print, but can be purchased online)
How Good is your Pol Limit Omaha by Stewart Reuben
Pot Limit and No Limit Poker by Ciaffone and Reuben

There is a section on PLO in Super System II, though IMO it is not worth buying the book just for that section.

Articles:
Rolf Slotboom has written many PLO articles, most of which can be found in the archives at Cardplayer.com

There are several websites with PLO strategy articles. Google is your friend.