I would give this a 3, but encourage anyone who isn't an advanced dev or naturally objective to shy away from it.
This book has a split personality:
Positive:
The tutorials in this book are fantastic, though some have issues in that they don't actually work, but those become fewer and fewer as the book progresses. This is arguably one of the most useful manuals in regards to how to actually perform tasks in InfoPath and anyone who can restrict themselves to just reading the point by points is going to get a lot out of it, and have a rock solid understanding of InfoPath.
Here's the other side...
Negative:
Pivotal points of this book label it as non-objective. The author has an issue in his writing in that he assumes his opinion and bias equals fact. He doesn't comment much outside of InfoPath, but two things stand out to me when he does:
1. He views InfoPath as a panacea for all SharePoint's ills
2. He hasn't touched office integration in 10 years, if ever, and doesn't have a clue what he's talking about in terms of integrating office documents with SharePoint data
He draw some conclusions that are not just wrong, they could stunt people's abilities as developers moving forward as he presents them as best practice when they're anything but.
Short summary:
The author has written a treasure trove of fantastic tutorials that are good for devs of all levels, but leaves his actual realm of expertise at a few key points to comment on things he by all appearances has little experience with.
Skip everything that could be taken as subjective, or take it with a serious grain of salt and this book is worth the money.