Riordan is at his best when he's writing action scenes and when he's respinning old Greek/Roman/world myths into people for Percy and his friends to meet, or events to happen, or places to go. I always particularly enjoy the way he makes them fit into the present day (like the Amazon warriors running the Amazon company in a previous book, and the Justin Bieber-ish fan cult around Narcissus in this book). Annabeth's encounter with her foe (no spoilers) at the end of the book is wonderfully tense and exciting. Luckily, there's a lot of action and new myth in The Mark of Athena. Unluckily, there's also a lot of romance and angsty teenagers, which is not something Riordan is particularly good at writing. A couple chapters worth of "Percy never wanted to let Annabeth out of his sight again" or "Piper looked at Jason and thought herself so lucky" (not actual quotes, just approximations) could have been cut to create a really fast-paced, exhilarating book. As is, the book is sometimes a drag to get through.