I just bought the early release of that exact book for $13.60, which was 60% off, because you get 60% off if you order $100 worth of prediscount ebooks.
When the book is finished you get the final version. It's mostly already finished.
"With Early Release ebooks, you get books in their earliest form — the author's raw and unedited content as he or she writes — so you can take advantage of these technologies long before the official release of these titles. You'll also receive updates when significant changes are made, new chapters as they're written, and the final ebook bundle."