Honestly I don't think there are any good PLO books these days. Jeff Hwang's Pot Limit Omaha and the following Advanced Series did give me a bit of insight several years ago, but honestly I read a few sections again a year or so ago and found them to be lacking in the context of the games I've played lately.
Although damn I just noticed you could get his first book for $4.49 including shipping used in "good" condition, so I guess you can't much go wrong with that. It just advises uber nitty play that, versus lagtards of the kind that you'll be playing against at any PCF meet up, will rate to make you profitable, but will be WAY less fun a strategy than you could otherwise employ (and lose only a tiny bit of that profitability).