I've got one of Jeff Hwang's books > "Pot-Limit Omaha Poker - The Big Play Strategy.
I've also got > "The Pot-Limit Omaha Book - Transitioning from NLHE to PLO" by TRI "SLOWHABIT" NGUYEN. (@ $300 it isn't exactly cheap but has some great reviews.... I just got it).
I've only just begun to read both so I'm unable to comment on what one(s) would be good but.. 'so far... so good'.
I'd also be interested to hear reviews of PLO books on here (kinda why I'm subscribing to this thread).