I recommend the Cappelletti book. It's not incredible, but it should help most players. IMO there really isn't a definitive O/8 book (even Ray Zee's book doesn't say much), and you'd be much better off getting your information from trusted online sources. I'd really recommend running some searches on
RGP. This place generally blows, but the search feature rocks - for instance, many respected poker authors frequent these forums and actively respond to Omaha/8 posts, along with many 2+2 regulars..