This is a book on the F# programming language.
On the surface of things, that is an intuitively obvious statement, given the title of this book. However, despite the apparent redundancy in saying it aloud, the sentence above elegantly describes what this book is about: The authors are not attempting to teach developers how to accomplish tasks from other languages in this one, nor are they attempting to evangelize the language or its feature set or its use "over" other languages. They assume that you are considering this book because you have an interest in learning the F# language: its syntax, its semantics, its pros and cons, and its use in concert with other parts of the .NET ecosystem.
The intended reader is a .NET developer, familiar with at least one of the programming languages in the .NET ecosystem. That language might be C# or Visual Basic, or perhaps C++/CLI, IronPython or IronRuby.