An interesting quirk of Scandinavian society is the concept of Jante Law. It knocks down standing out and being individual, in favor of communal harmony. It's typically used in a negative context to lament restrictions and lack of risk taking within Nordic society but the flip side of the Jante coin is "lagom": the idea and ideal of having just the right amount of something.
Sinatra Up and Running is, second to K&R, the most "lagom" technical book I've read. At a mere 102 pages you may wonder whether it's worth buying - it is. Unlike most technical books - yes, including mine - it skips the waffle and provides a perfect level of detail going through from what Sinatra is, to how it works, and on to an example project that covers just 13 pages. Don't be fooled, though, this isn't one of those tiny format O'Reilly handbooks; it's a regular, full size book - just a thin one!
It's a good book and well written. I enjoyed it and picked up or was reminded of quite a few interesting bits and pieces. I'll probably refer to it from time to time. If your Sinatra experiences are rather on and off or you've not played with it for a while, it's a great, well-paced introduction. If, however, you're already a Sinatra guru and/or working with Sinatra on a day by day basis and have all of the main patterns memorized, there's not a great deal you're going to get out of it. Buy it to be a completionist or to support the authors, but if you want a book demonstrating in depth how to integrate Sinatra with everything or how to big giant Web applications, this isn't for you.
Inexperienced Rubyists may also find the book's direct no-nonsense style intimidating. If you know what a code block is, you're good to go. This may seem like a bizarre observation to most Rubyists, but I've encountered many beginners who've wanted to "build a Web site" and immediately leapt into an advanced Rails book, only to be confused. If you're still new to Ruby, read The Well Grounded Rubyist or Beginning Ruby first.
And I'm going to stop here, because that would be lagom :-)