AUTHOR'S PURPOSE
The author, Rick Riordan, wrote the Mark of Athena for ,one obvious reason, to entertain. This book is a fantasy book, no offence to the Greek believers, and is very fictional. So it cant be used to inform the readers unless Rick's intention to write this book was to inform us more about Greek and Roman gods and mythology. But it obviously isn't cause then there would be no reason for a story board, he could have simply wrote a Non-Fictional book. Also he didn't write this book to persuade anyone to go find Arachne and avenge Athena. So it only leaves one purpose: to entertain!