Relation between tables
It seems to me that in a case like this you need a linking table that will allow for that many to many relationship.  Something like

Create Table tblBookWriter
(tblBookWriter_ID int
, Book_id --(FK to Book table-Book_Id)
, Writer_id --(FK to Writer table-Writer_Id)
);


With this you can add have a book with as many writers as it needs and a writer can have authored as many books as is needed.  Something like that should get it for you anyway.

If you do a web search for "database many to many linking table" you will see all kinds of results.  The first is a wiki link for a Junction Table