调用Json数据将返回未定义的“

当我从php文件调用json对象时，它返回未定义.我可以看到所有写入alert(data)的数据，但是当我编写alert(data.books)时，它返回未定义的值.

When I call json object from php file it returns undefined. I can see all data writing alert(data) but when I write alert(data.books) it returns undifined.

$JSON = '

{
  "books": {
 "book1": "firstbook",
 "book2": "secondbook"
  }
  }
';

我用jQuery来称呼

and I call it with jquery

jQuery('#login').live('submit',function(event) {


$.ajax({
    url: 'lib/login.php',
    type: 'POST',
    dataType: 'json',
    data: $('#login').serialize(),
  success: function(data ) {

alert(' ' +data.books);

  if(data.books.book1){  
alert("OK"); 
}else
{
alert("error");   
}
}

 });

   return false;


});

编辑 这是它返回alert(data)的方式

EDIT This is how it returns alert(data)

    {

  "books": {

  "book1": "firstbook",

  "book2": "secondbook"

}

}