Older, and not free -- C# Programming Language, The 2nd Edition by Anders Hejlsberg
Chapter 1 of this book is a short and nice introduction to C# for programming language experts. However, the following chapters are not easy to understand even for experts. You can download a newer version of "C# Language Specification" (a standard from ECMA but whose content is mostly the same as this book) free. Do you still want to buy this book?
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Related links: C# Programming Language, The 2nd Edition@Amazon.com, ECMA's C# Language Specification.
