Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Apple Release iTunes 10.5.3 To Public –
iTunes 10.5.3 allows you to sync interactive iBooks textbooks to your iPad. These Multi-Touch textbooks are available for purchase from the iTunes Store on your Mac or from the iBookstore included with iBooks 2 on your iPad.
iBooks textbooks are created with iBooks Author — now available as a free download on the Mac App Store.
Earlier today we seen Apple release iBooks 2.0 , iBooks Author and an iTunes U iOS App. Now we have just seen Apple release iTunes 10.5.3 to the public. iTunes 10.5.3 enables you to sync textbooks to iPads. You can expect to see iTunes 10.5.3 in your software update soon.
You can download it HERE