First you must determine how you want to publish your book, which is what you should have done before you got this far.
Are you going to:
1. Print it to your desktop printer.
2. Have it printed and bound by a short run (less than 500 copies) commercial (inkjet/Xerox) printer you find on the Internet
3. Have it printed and bound by a commercial Offset (printing press) printer
4. Make an ePub (digital) book
5. Make a PDF book, locked or unlocked
6. Is it going to be free or are you going to charge for it, if so how do you expect to get paid and what do you think is reasonable?
Any physical printed books will be very expensive.
Any ePub or PDF or physical book will almost certainly make no money and will very likely cost you a lot instead.
If you still want to go ahead:
In iBooks Author:
Menu > File > Export > PDF or Text
The PDF will give you a digital version of the book which you can lock if you wish and/or publish through the Adobe Digital Management System which means users can pay to download it and not be able to steal it.
The Text will only give you the Text for reuse elsewhere, say in Pages. The images and anything else you have will need to be gathered elsewhere. You can copy them from iBooks Author and resave them from Preview .app.
If you are going to use Pages (which version?) to create your book, you need to work out what it is your end book is going to be (see list above) and work back from that.
Peter