"dereferencing pointer to incomplete type" error. Any advice?

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define M 100
#define FLUSH while(getchar() != '\n')

typedef struct node *nd;

typedef struct 
{
       char bkNumber[5];
       char bkTitle[M];
       char bkAuthor[M];
       int bkCopyright;
       
}BOOK;

typedef struct
{
        BOOK rec;
        nd right;
        nd left;
}NODE;

BOOK getData(void);
void addNode(BOOK, nd *);

int main(void)
{
    nd root;
    BOOK data;
    int choice;
    
    printf("1] Add new book record \n2] Edit book title \n3] View all the books");
    printf("\n4] View a specific book \n5] Delete a book \n\n");
    printf("Choice: ");
    scanf("%d", &choice);
    
    system("cls");
    
    switch(choice)
    {
        case 1:
             data = getData();
             addNode(data, &root);
             break;
    }
    
    return 0;
}

BOOK getData(void)
{
     BOOK d;
     FLUSH;

     printf("Book Number: ");
     scanf("%4s", d.bkNumber);
     
     printf("Book Title: ");
     scanf("%s", d.bkTitle);
          
     printf("Author: ");
     scanf("%s", d.bkAuthor);

     printf("Copyright Year: ");
     scanf("%d", d.bkCopyright);
     
     return d;
}

void addNode(BOOK data, nd *root)
{
     nd temp, tp, tp1;
     
        temp = malloc(sizeof(NODE));
        
        strcpy(temp->rec.bkNumber, data.bkNumber);
        temp->right = NULL;
        temp->left = NULL;
        
        if(root == NULL)
                *root = temp;
        else
        {
            tp1 = *root;
            
            while(tp1 != NULL)
            {
                      tp = tp1;
                      if(temp->rec <= tp1->rec)
                          tp1 = tp1->rec;
                      else
                          tp1 = tp1->right;
            }//end of while
            
            if(temp->rec <= tp->rec)
                      tp->left = temp;
            else
                      tp->right = temp;
        }//end of else
        
        tp = NULL;
        tp1 = NULL;
        
        return;
}//end of addNode