The Mark of Athena, by Rick Riordan, is the third installment to the Heroes of Olympus series, which is a companion/continuation of the popular Percy Jackson and the Olympians series. In this book, Percy Jackson and Jason Grace join forces with five of their fellow demigods to continue the battle against Gaea, the Earth Mother. Each demigod is tested to the limit as they face gods and trials on their quest, but also learn more about themselves and their relationships with each other.
Although the book has some slow moments and is full of cheesy lines, that's part of what readers love about Rick Riordan's books. The book is quite long (608 pages), but plenty of action sequences will keep readers entertained and turning pages. If you have readers that love the Percy Jackson series, the Heroes of Olympus series is a must-read.