I have a bookshelf groaning with books on C# and agile software development. This is without doubt the best I've read. Robert Martin has somehow managed to distil his experience of many years into one cohesive tome, which is easy to read and provides practical advice on all aspects of development - agile, TDD, UML and design patterns.
As for the previous reviewer who gave this a ludicrous 1 star review and suggests the GOF as a better alternative, I would suggest that he is either an abstract genius, or has never actually read the GOF stuff and is just name-checking this in a poseur like fashion!
For the rest of us, this is probably as good as it gets.