Sinatra is a simple and powerful web server framework, and this book is worth reading if you're interested in Sinatra. However, this book is not as comprehensive as the O'Reilly book on Rails, nor is it as polished as other O'Reilly books.
Specific criticisms: - There are some silly mistakes in the text, which better proof-reading and editing would have corrected. I think this probably extends to mistakes in example code. - Some useful features of Sinatra aren't really explained - for example layouts are mentioned but not fully explained. - The book gives a glimpse of how Sinatra works, but it doesn't fully explain Rack or the Ruby magic that Sinatra uses. I found myself wanting a more comprehensive explanation. Alternatively, I would preferred a full description of how to use Sinatra with less of how it works. Instead the book leaves me thinking "it's rather complicated, I don't think I really understand it". - It would be useful to cover other Ruby tools that work well with Sinatra.
It's all in here, you just need to find it. A good reference after going through a gentler introduction. I suspect this would be the well thumbed reference book on my shelf, if it wasn't for me having the kindle version.