To get the update in iBooks, delete the current version of the book on your iPad, go to the iBookstore, and re-download. Apple will already remember that you purchased it and you will not have to pay again. In the process, you will lose annotations that you made to the prior version. Sadly, there is no delta update mechanism for books. We are pioneers together.
There’s also a PDF version, created using iBooks Author, which (unlike the iBooks version) you can read, search, and persistently annotate from your Mac. Another interesting format issue is that the PDF version is separable—the 800+ MB of screencasts are in a separate folder. I’ve archived the videos but don’t plan to store them on my Mac’s internal SSD. The iBooks version is presumably all one unit. You can’t access the text without the videos filling up your iPhone or iPad. And if you delete the book to free up space, you risk losing your annotations the next time the book is updated.