Probably one of the best entry-mid level reference books on the C# language i’ve had the pleasure of reading. CLR via C# is a sizable volume that hammers out great concepts and techniques of OOP by showing how they operate in the CLR.
If you are an experienced (C#) developer, the first 25 chapters will go amazingly fast- covering topics like primitives (the data type, not the developer sitting in the adjacent pod) methods, and basics of OOP.
However, what drew me to this book was Part 5 (Chapters 26-30) which covers threading. Probably the best book on threading you can get your hands on. Lots of (relevant) code examples and thorough explanations.
If you’re looking for a developers reference for C#, this is a great book to have at your disposal.
You can read more about it here.