Why should you care about it?
This is like a complete C# book that covers all the language features that is released by Microsoft for each version of the language. Its free, works like a quick reference guide without all the mumbo-jumbo of other books delving with examples.
But aren’t we on .Net 4.5?
Yes, C# 5.0 is .NET 4.5 – confusing I know. The language specifications and the framework are two different things. .NET 4.5 is part of VS 2012 which enable you to write C#5
What’s new in it?
Well, a lot. First there is the async functionality built into the language. Second most important is the CallerInformation that enables you to see, well, caller info without using reflection.