These are the books about PLO I know about (and have):

"How Good Is Your PLO?" by Stewart Reuben. Very loose high stake deep money PLO between regulars at the Victoria Casino in London. Interesting reading made in quiz form but the game has little to do with shallow money online PLO.

"Super System 2" Nice chapter by Lyle Bernan. My favourite, covers most of the PLO concepts.

"Omaha Holdem Poker" by Bob Ciaffone. A rather thin book that also covers other forms of Omaha. Written like small essays.

"Improve Your Poker" by Bob Ciaffone. Includes som PLO.

"Pot-Limit And No-Limit Poker" By Bob Ciaffone and Stewart Reuben. Same as above.

Also read what Rolf Slootbom writes about PLO. The articles can be found at pokerpages.com

See ya at the tables!