ORA-02267: column type incompatible with referenced column type

SQL> CREATE TABLE Selling
2 (Username char(10) PRIMARY KEY,
3 BookID number(5),
4 Title char(30),
5 Authors char(50),
6 Edition number(2),
7 ISBN number(13),
8 CourseID number(5),
9 Price number(4,2),
10 BuyerUsername char(10),
11 BuyerCreditCard# number(16),
12 TransactionDate number(8),
13 UNIQUE(Username),
14 UNIQUE(BookID),
15 UNIQUE(ISBN),
16 UNIQUE(CourseID),
17 UNIQUE(BuyerUsername),
18 UNIQUE(BuyerCreditCard#),
19 FOREIGN KEY(Username) REFERENCES Seller,
20 FOREIGN KEY(BookID) REFERENCES Book,
21 FOREIGN KEY(Authors) REFERENCES Book,
22 FOREIGN KEY(Edition) REFERENCES Book,
23 FOREIGN KEY(ISBN) REFERENCES Book,
24 FOREIGN KEY(CourseID) REFERENCES Course,
25 Check(Price>0));
FOREIGN KEY(Authors) REFERENCES Book,
*
ERROR at line 21:
ORA-02267: column type incompatible with referenced column type

SQL> describe book;
Name Null? Type


BOOKID NOT NULL NUMBER(5)
TITLE NOT NULL CHAR(30)
AUTHORS CHAR(50)
EDITION NUMBER(2)
ISBN NOT NULL NUMBER(13)

SQL>

What do I do wrong now please help