VS2015 add reference for Class Library

I have created a project in VS2015, structure as below:

Solution1

• BookStore.ClassLibrary1 => Class Library (Package)
• BookStore.ClassLibrary2 => Class Library
• BookStore.Web => MVC5

In BookStore.Web, I can reference BookStore.ClassLibrary2, but fail to referenceBookStore.ClassLibrary1.

It shows an error "A reference to 'ClassLibrary1' could not be added."

My question is how to reference a Class Library (Package) in VS2015? Thank you so much!