Go4Expert - undefined reference Error!

Would someone help me understand why I keep getting the error:
"In function main: [linker error] undefined reference to readBook(BookData)"

I have spend hours trying to fix it, but no luck :/

Thanks in advance

main.cpp :

Code:

#include <iostream>

#include <string>

#include <iomanip>

using namespace std;

#include "BookData.h"



// FUNCTION PROTOTYPES:

void bookInfo(BookData);

void readBook(BookData& book1);



int main(){

    BookData book1, book2;

    cout << "\nEnter the data for the first book:\n";

    readBook(book1);

    cout << "\nEnter the data for the second book:\n";

    readBook(book2);

    cout << "\nDisplay the data for the first book:\n";

    bookInfo(book1);

    cout << "\nDisplay the data for the second book:\n";

    bookInfo(book2);

    system("pause");

    return 0;

}

void bookInfo(BookData book)

{

     string temp;

     cout << left;

     cout << "   " << setw(22) << "ISBN:" << book.getIsbn() << endl;

     cout << "   " << setw(22) << "Title:" << book.getTitle() << endl;

     cout << "   " << setw(22) << "Author:" << book.getAuthor() << endl;

     cout << "   " << setw(22) << "Publisher:" << book.getPub() << endl;

     cout << "   " << setw(22) << "Date Added:" << book.getDateAdded() << endl;

     cout << "   " << setw(22) << "Quantity-On-Hand:" << book.getQty() << endl;

     cout << "   " << setw(22) << "Wholesale Cost:" << "$" << fixed << setprecision(2) << book.getWholeSale() << endl;

     cout << "   " << setw(22) << "Retail Price:" << "$" << book.getRetail() << endl;

     temp = book.isEmpty()?"yes":"No";

     cout << "   " << setw(22) << "Empty?" << temp << endl;

}

void readBook(BookData& book1)

{

     string tempS;

     int tempI;

     double tempD;

     

     cout << "Enter the Title: ";

     cin >> tempS;

     book1.setTitle(tempS);        

     

     cout << "Enter the ISBN: ";

     cin >> tempS;

     book1.setIsbn(tempS);

     

     cout << "Enter the Author: ";

     cin >> tempS;

     book1.setAuthor(tempS);

     

     cout << "Enter the Publisher: ";

     cin >> tempS;

     book1.setPub(tempS);

     

     cout << "Enter Date Added: " ;

     cin >> tempS;

     book1.setDateAdded(tempS);

     

     cout << "Enter Quantity on Hand: ";

     cin >> tempI;

     book1.setQty(tempI);

     

     cout << "Enter Wholsale Price: " ;

     cin >> tempD;

     book1.setWholesale(tempD);

     

     cout << "Enter Retail Price: " ;

     cin >> tempD;

     book1.setRetail(tempD);

     

     cout << "Enter 1 to insert the book or 0 to remove: ";

     cin >> tempI;

     while (tempI > 1 || tempI < 0)

     {

           cout << "Wrong entry. Try again: ";

           cin >> tempI;

     }

     if (tempI == 0)

         book1.insertBook();

     else

         book1.removeBook();     

}

BookData.cpp :

Code:

#include <string>



class BookData

{

    private:

        string bookTitle;

        string isbn;

        string author;

        string publisher;

        string dateAdded;

        int qtyOnHand;

        double wholesale;

        double retail;

        bool empty;

        

    public:

        BookData();

        void setTitle(string object);

        void setIsbn(string code);

        void setAuthor(string auth);

        void setPub(string pub);

        void setDateAdded(string dateAdd);

        void setQty(int quant);

        void setWholesale(double price);

        void setRetail(double temp);

        bool isEmpty();

        void insertBook();

        void removeBook();

        string getTitle();

        string getIsbn();

        string getAuthor();

        string getPub();

        string getDateAdded();

        int getQty();

        double getWholeSale();

        double getRetail();

};



BookData::BookData()//Default Constructor

{

  qtyOnHand = 0;

  wholesale = 0;

  retail = 0;

  empty = true;

  bookTitle = isbn =  author =  publisher = dateAdded = "";

}



void BookData::setTitle(string object)

{

    bookTitle = object;

}



void BookData::setIsbn(string code)

{

    isbn = code;

}



void BookData::setAuthor(string auth)

{

    author = auth;

}



void BookData::setPub(string pub)

{

    publisher = pub;

}



void BookData::setDateAdded(string dateAdd)

{

    dateAdded = dateAdd;

}



void BookData::setQty(int quant)

{

    qtyOnHand = quant;

}



void BookData::setWholesale(double price)

{

    wholesale = price;

}



void BookData::setRetail(double temp){

       retail = temp;

}

bool BookData::isEmpty(){

     return empty;

}

void BookData::insertBook(){

     empty = false;

}

void BookData::removeBook(){

     empty = true;

}

string BookData::getTitle(){

       return bookTitle;

}

string BookData::getIsbn(){

    return isbn;

}

string BookData::getAuthor(){

       return author;

}

string BookData::getPub(){

        return publisher;

}

string BookData::getDateAdded(){

       return dateAdded;

}

int BookData::getQty(){

    return qtyOnHand;

}

double BookData::getWholeSale(){

       return wholesale;

}

double BookData::getRetail(){

       return retail;

}

DataBook.h :

Code:

#ifndef BOOKDATA_H

#define BOOKDATA_H

#include <string>



class BookData

{

    private:

        string bookTitle;

        string isbn;

        string author;

        string publisher;

        string dateAdded;

        int qtyOnHand;

        double wholesale;

        double retail;

        bool empty;

        

    public:

        BookData();

        void setTitle(string object);

        void setIsbn(string code);

        void setAuthor(string auth);

        void setPub(string pub);

        void setDateAdded(string dateAdd);

        void setQty(int quant);

        void setWholesale(double price);

        void setRetail(double temp);

        bool isEmpty();

        void insertBook();

        void removeBook();

        string getTitle();

        string getIsbn();

        string getAuthor();

        string getPub();

        string getDateAdded();

        int getQty();

        double getWholeSale();

        double getRetail();

};

#endif