Pattern Recognition and Machine Learning is by Christopher Bishop (Chief Research Scientist at Microsoft Research Cambridge, and Professor of Computer Science at the University of Edinburgh).
It's a heavy read on machine learning algorithms in that it is very math intensive. It would not be too far off to call this an applied math textbook rather than a computer science textbook, and it is definitely aimed at the graduate level student with a good amount of mathematical maturity.
Having said that, if you are a machine learning researcher in need of learning all sorts of mathematical details of the algorithms you are using, this book is a good place to start. It is by no means a complete reference, but certainly a good introduction at the graduate level.
If you are an undergraduate student, there are other textbooks available that might be more suitable (such as Artificial Intelligence: A Modern Approach, which I also reviewed). If you just want cookbook style text on the libraries you could use to employ machine learning algorithms, there are probably other resources that would be more suitable.
If you are starting out in researching machine learning algorithms, or are using it as a large part of your computing science research, this is the right book. Just be prepared to learn and use a lot of math!