You may have accidentally paused the download. Click on the iBooks icon again to resume the download. Check if the iBooks app's name is now called "Paused." This means you paused it.
I also have the same problem on an iPhone 4S, having just download IOS6. The iBooks app is still downloading after 1hr & I see no means of aborting/cancelling the download of iBooks & trying a alternative app.
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