iBooks Author is Mac OS X application which will help an author or publishers to deliver their knowledge into eBook which complied neatly and listed in iBookstore. Of course you can set your written and published book for free or paid eBook.
And your iBook will be exposed to thousands of thousands iOS device users who search for educational book for their need.
iBooks Author application is currently available on Mac App Store, that you can use to build an powerful and interactive book. The main purpose of Apple launched this app to help students getting books for their educational need, and as for the book authors to provide them with textbooks.
In other words this application is connecting you as author and the student in one single media -- iBookstore on iPad.
If you are interested with iBooks Author, you can head to Mac App Store and download iBooks Author.