i have problem with creating a custom markup extension

i am trying to learn Xamarin by reading Ceating mobile apps with Xamarin Forms. Page 210 explain how to link Xamarin.FormsBook.Toolkit to CustomExtensionDemo project. I follow instructions(Include a link to the library project in your application solution, and add a reference to that library project in the common PCL project.) but Xamarin studio says FormsBook doesn not exist in the namespace. I have created Markup extension and CustomExtensionDemo project. there is also link in project. new Xamarin.FormsBook.Toolkit.HslColorExtension(); nevertheless it gives error message. i don't know how to reference to that library project in the common PCL project perhaps