Rick Riordan is the #1 New York Times bestselling author of tons and tons of novels for tweens.
The Trials of Apollo by Rick Riordan is a fantasy/mythological book where the Ancient Greek god Apollo was cast down from Olympus, which is the home of the gods, by Zeus. Apollo became mortal and is now called 'Lester Papadopoulos.' he must complete multiple deadly trials to become a god again. But, according to Zeus, he needs a master. Meg McCaffery is Lester's master,and he is forced to do even simple commands from Meg, such as 'be quiet' or 'go away.' He has the help of the young demigod, Percy Jackson, to guide him to Camp Half-Blood which is a safe place for demigods and really any target for Ancient Greek monsters.
I would suggest reading the Percy Jackson series and the Heroes of Olympus series before the Trials of Apollo - as the Trials of Apollo follow on from these. I believe the Trials of Apollo - The Hidden Oracle by Rick Riordan is an amazing book because you always want to know what will happen next. I wouldn't change anything about it. I would recommend this book for children aged 8 - 13. It is an exciting, thrilling and humorous book.