Modify a data member in a const method

The class Book below has a const method - Author() - that returns the name of the book's author:

class Book {
 public: 

  const std::string& Author() const {
     if( author_.empty() ) {
       // Lazy load
       author_ = loadAuthorFromDb(); // ERROR !!
     }
     return author_;
  }

  //.. more public interface
 private:
  std::string author_;
  //.... more data members
  std::string loadAuthorFromDb() const; 
};

As the author's name is not frequently accessed, we are loading it from database only if it is necessary. The method Author() does a one time lazy load of author's name from database when it is called the first time.

We are getting a compilation error when assigning to data member author_ because the method Author() is const. Which one of the following modifier keywords we should use for data member author_ to avoid this compilation error?