I'm the author. I wanted to give a heads-up that the book has been through four free updates, but there will be another free update after the next Helix firmware becomes available. To obtain the update, all you need to do is download the book again. Whatever you download will always be the most current update. Updates cover the features from new firmware, but they also include more applications for new and existing features.
To find out what people think, check out the reader reviews on the Sweetwater site. They've all been extremely positive except for a couple people who expected it to be a "Helix 101" book for beginners. It is not. It's written to cover what's not in the manual, describe applications (e.g., virtual "Nashville tuning"), and provide technical information about the blocks that isn't available anywhere else. The original reason for writing the book was because the manual didn't describe what the block parameters actually do.
The only other caveat is the presets. Not all are meant to be "plug and play." Some are simply designed to show a particular technique, so you load them to experiment with something described in the text. Then using what you learned from reading the book, you can modify them for your specific needs.
Scroll down the book's landing page and you'll see the contents toward the bottom. The topics that are covered will help you decide if you want to go deep into maximizing the Helix's potential, or need something more introductory.