/in/foo.c: In function 'main':
/in/foo.c:7:2: error: 'book' undeclared (first use in this function)
  book[n+1][n+1]={0};
  ^~~~
/in/foo.c:7:2: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:7:17: error: expected expression before '{' token
  book[n+1][n+1]={0};
                 ^