It's assumed that Apple will sell books through iTunes with a revenue-sharing model similar to what's currently behind the App Store. The report states that the publishers are keen to control pricing and retain access to the files themselves. It's interesting to note that Amazon recently (as in yesterday) announced a revised and similar business model for the Kindle, scheduled to go into place on June 30th.
Now, just because Apple is in talks with all six publishers doesn't mean each deal is a lock. At least not before next week's dog-and-pony show. Expect a demo, some partner announcements (with select CEOs briefly sharing the stage) and a fanboy-crushing delay of 4-5 months.