Extraordinary is a powerful word so it drags in the reader into his book.

Extraordinary is better than excellent and brilliant because extra gives ordinary more power. Ordinary is just like good so he added extra witch gives it more power so extra means that they are extra special to them.

Thunder is loud so it brings in the reader thunder describes the heart and gives it more of a hook to the reader.
Thunder is really loud so that means that the heart is pumping really fast and really loudly thunder is a good adjective to interest the reader to keep on reading.
skellig quotations