Essential C# 6.0, by Eric Lippert and Mark Michaelis – cover the C# language up to the 6.0 update of last year.
Coverage and examples are pretty good. One thing I liked is that any extensions to the original language are clearly marked with a version number. If you get the book to update your knowledge, there is also a nice index for each version.
Given how little changed in version 6.0, it'd be silly to buy the book though – there are plenty of blog posts covering this. If you get it for free (maybe from Safari or something similar at work), then it might be a good idea.
Overall, pretty good coverage, although I'd recommend C# in a Nutshell instead – it felt a little more complete and had more developed examples.
