ROR ERROR:Couldn't find Book without an ID

Here are two solution :

1:
change
<form action="../update/" method="POST">
to
<form action="../update/<%=@book.id%>" method="POST">

2:
change
<input id="book_id" name="book[id]" size="30" type="hidden" value="<%=@book.id%>"/>
to
<input id="id" name="book[id]" size="30" type="hidden" value="<%=@book.id%>"/>