can not convert value type "string?" to expected argument type "inout string"

You need to write it like this instead:

self.displayResultLable.text =  self.displayResultLable.text! + title as! String

It´s because of the left side is an optional and the right side is not and they don´t match. That´s why you need to write label.text = label.text +...

I can also suggest you to change your if let to this instead:

if let title = books.valueForKey("title") as? String {
   self.displayResultLable.text = (self.displayResultLable.text ?? "") + title
}