For information on getting a book, go to Greenfoot's page on the book. BTW, the only PDF-formatted information I found on the site was the Greenfoot API.

Safari Books Online appears to carry the Greenfoot Book (http://greenfoot.org/book) and if you have a public library account, some libraries appear to have a relationship with Safari Books Online so that you can use that as a way to read / reference the book as needed. There's also an option to buy the e-book.