Don't know much about Ray Zee. His book is oldish, and while touted somewhat highly by more expert-level players, gets mixed reviews.
If you're just learning you can't go wrong with this. I got this book, have read most of it. It's perfect for beginner/intermediate. Only problem I have with it, it seems most hands are in the $10 - $20. A beginner playing this at Full Tilt would probably get eaten alive. Anyway, It's only once you get to the intermediate level (which I consider myself even though I play the micros mostly) that you can pick apart his (thankfully) far & few between high risk and probably ill-advised plays:
Amazon.com: Pot-Limit Omaha Poker
(9780818407260): Jeff Hwang: Books
Hwang pays homage to Bob Ciaffone, who's book he took off from. But go to Amazon, and read the excerpts and reviews.
More recent (and very advanced) book from Hwang. I haven't read any, but has garnered great reviews:
Amazon.com: Advanced Pot-Limit Omaha: Small Ball and Short-Handed Play (9780974150291): Jeff Hwang: Books (http://www.amazon.com/gp/product/0974150290/ref=s9_simi_gw_s3_p14_i1?pf_rd_m=ATVPDKIKX0DER&pf_rd_s=center-3&pf_rd_r=1S4WVE1S6KERSQQSG6YK&pf_rd_t=101&pf_rd_p=470938811&pf_rd_i=507846)
Lastly, and I never even heard of this one or its author(s). Came out last July, and good reviews thus far. You can also search inside this book, and I'll be doing just that after this post :
Amazon.com: Pot Limit Omaha 8 Revealed (9781442197633): Dan Deppen, Anna Paradox: Books (http://www.amazon.com/gp/product/1442197633/ref=s9_simi_gw_s3_p14_t2?pf_rd_m=ATVPDKIKX0DER&pf_rd_s=center-3&pf_rd_r=1S4WVE1S6KERSQQSG6YK&pf_rd_t=101&pf_rd_p=470938811&pf_rd_i=507846)