So you probably noticed that a captcha will have one word that is wavy, and another word that is more straight-fowarded. The wavy ward is used as the validation, while the normal word is used by Google for Google books. When google's algorithm can't recognize a word, this word is copied in a captcha and you are used by Google to recognize this word. So you can basically type anything you want for the easily-readable word as long as you get the wavy one correct.
The more you know.