Visual Studio is the default IDE for writing C# code, although it's not the only option when it comes to writing apps with Xamarin.
There are other options, such as Xamarin Studio for Windows and Xamarin Studio for Mac if you have a Mac. Xamarin Studio for Windows, though, is no longer supported by Microsoft and they encourage developers to use Visual Studio instead. For developing on the Mac, you can use Xamarin Studio for Mac.
In this book, we'll be covering Xamarin development using Visual Studio on Windows, since it is the best IDE available for code development using C# and provides great IntelliSense support.