Cappeletti has a pretty good book (from what I've heard).
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'So we go adjust the flow and everybody should know, but in case it erase remember me tell you so, No matter how we scatter in different lands you have turn and learn and try understand'- Tony Rebel
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Ray Zee also has a good one that covers both O8 and stud hi/lo. I have read the one by Cappeletti and it is a good one. One that I have not read but want to pickup in the future is a book on PL by Ciaffone.
Ken Warren has "Winners Guide to Omaha" but it is very basic.
Bob Ciaffone has an Omaha Poker book. It's was written awhile ago and was out of print, but it has been re-released. Do a search on amazon. It's good, but short book. I also heard Rolf is coming out with a book soon, but not sure when's its being released.
I like Bob's writing style, very short and consice. He and Stewart Ruben have another very good book called Pot Limit and No Limit Poker. Great concept book.
Winnig Omaha/8 Poker By Mark Tenner and Lou Krieger
i have just bought this book on amazon.i havent received it yet so i cant tell you how good it is but on amazon you can look at the table of contents and it has many many chapters and sections inside the chapters.look it up i think it will def help you out with you omaha 8 game.