I suggest you echo the query to see whether it is what you are expecting. Modifiy the part where you construct the query like this:
// put the query in a variable
$q = "UPDATE books SET Title ='$title_save', Price ='$price_save', Stock='$stock_save' WHERE Code= '$code'";
// submit query using the variable
mysql_query($q) or die(mysql_error();
// add this to display the query
Now you will be able to see if the query gets constructed OK and you can copy it to phpmyadmin or mysql client and test it.
Two other notes:
your form has no </form> tag. It should be after </table> tag.
your form is missing an action attribute (I think it is required)