C# in depth is written by Jon Skeet, a software engineer currently working at Google. He is known for being an extremely active user on Stackoverflow, having an odd fascination in the field of date and time (he is one of the Noda Time library authors), and being very interested in the C# language.
As the C# language has evolved and new versions have been released, so have new editions of the book been published. The latest edition, the fourth, covers C# 1 – 7, and a little bit on the upcoming C# 8. It is pretty much a history book of the C# language with deep dives into the most important changes and features.
I really want to stress that this is a book on the C# language and not the .NET Framework. Hence, it covers how the syntax, keywords, and language itself has evolved. It is also not a book for someone starting out with C# wishing to learn how to write Hello world, it is for the intermediate to advanced developer.
One reflection I had when reading this book is that Jon sometimes writes as he is explaining concepts to someone that has very little experience with C#, and in the next paragraph he writes for someone with deep knowledge of the language. Many parts of the book was on things I already knew quite well and I could just skim through them, and some parts I had to read really slowly to be able to follow along.
I also think that he sometimes takes it a bit too far, even though the title of the book is C# in depth. I never thought it was possible to write so much on tuples… Anyway, most sections of the book is interesting, well written, and well explained.
Summary: This is not a must read. You can be a great C# developer without having read it. But if you are interested in the history and evolution of the C# language, and wish to gain deeper understanding of the different parts that make up the language, then this book is for you.