By clicking the Preview button in the toolbar, you can see exactly how your iBook would look like on the computer you are using.
You can also directly plug in your iPad to your computer to get a live preview of your book on your iPad - very cool!
There are multiple ways you can save your book.
By simply clicking File > Save, you are saving your document as an iBooks Author document, which is the editable file. In order for students to read your book, we have to instead click on File > Export.