/in/ex6.c:5:1: error: unknown type name 'book'
 book *findBook(book *head, int number);
 ^~~~
/in/ex6.c:5:16: error: unknown type name 'book'
 book *findBook(book *head, int number);
                ^~~~
/in/ex6.c:7:1: error: unknown type name 'book'
 book *removeBook(book *head, int number);
 ^~~~
/in/ex6.c:7:18: error: unknown type name 'book'
 book *removeBook(book *head, int number);
                  ^~~~