I had mixed feelings about this book. The thing is, I can't really figure out who it is for.
I really enjoyed the first few chapters. They give you a nice overview of the platform with everything you need to know to build and deploy scalable applications. They also nicely explain why things need to be done the way they are done. They don't go into too much detail, but I was satisfied with the information I got.
Then it goes through all kinds of add-ons with detailed step-by-step screenshots on how to set them up. It gets a bit weird when it shows how to build and deploy sample applications in about 10 programming languages and frameworks. There is also a lot of repeated information in the book, which might be fine for some readers, but I found it a bit annoying.
It seems like the book targets beginners, who are building their first applications. But I felt like some parts wouldn't make too much sense for people who haven't yet developed and deployed a web application. And anyway, does it really have to go through every language and framework out there?
All in all, I think the book definitely gives you the knowledge you need to understand how Heroku works. Developing for the Heroku platform requires some special architectural and other considerations, which the book covers nicely. If you are a more experienced developer, you will probably find yourself skimming through many chapters, but that is fine as long as you learn what you need.