This is Andy Lindeman's book (Philip De Smedt produced a similar one). Much better than the official RailsGuide on the topic. And very useful, especially when the alternative is searching for blog posts. I hope he earned enough from the book to want to do more. We need this kind of quality documentation.
Yup - I wrote a similar book (Google is your friend if you want to find it - don't wanna steal publicity here). I haven't sold that many but the process of writing it and giving support has been really cool. I learned a ton in just writing the book and currently I'm working on my next one (on AngularJS + Rails). Funnily enough, I've also been thinking about giving away Upgrade to Rails 4 for free through Leanpub. Might do that sooner or later.
If you're in a position to buy this book and support the author you should do so. Our team used it to do the Rails 4 upgrade, and we EASILY saved greater than the minimal $75 cost of the book.
Cool.
There was, of course, always the free Rails-provided upgrade guide:
I want to redo the cover. Think thats kosher? I have actually wanted to do it for a while now, before it was even open source. Too awesome of a book for that so-so cover.