I'm not really a "PLO guy" but I liked Pot-Limit Omaha Poker The Big Play Strategy by Jeff Hwang. It's not too dense and if you are a solid NL-holdem player I feel the concepts here can get you comfortable playing omaha as far as knowing what and why hands are playable and what you are looking to do with flops and when to bomb pots vs pot control and stuff. I feel there is a lot in this game that is counter intuitive if you go into it from a NL hold'em perspective. they are two completely different games. I feel like I like NL holdem more but definitely learned tons from this book.