I've been working with a Scrum based project for a few months now, and in preparation read Planning Extreme Programming (XP) (as I'm overseeing from a process/responsibilitiy angle rather than actively developing on it) which I reviewed at the time as being
Well written, easy to follow book with great examples. Lots of ideas to try and hints and tips of common pain points. Have been adapting our current agile-ish process based on my readings so far, and will continue to do so to ensure we have a process that works for out team/product.
I stumbled across The Scrum Guide last week, and have just spent the past hour or so reading it and found it a really valuable use of my time. It is a well-written, simple and straightforward guide and well worth a read, especially as it manages to convey a lot of information in a comparably short document (just 17 pages in the English version). I've just sent it to the rest of the team and heavily suggested that they read it too.