When a popular author decides to allude to your culture in his book, it makes you feel fly, bursts your brain actually. Rick Riordan's use of Yoruba terms and general reference to Yoruba culture took me completely by surprise. As Yoruba culture is rich in mythology, I have a secret hope that Riordan's next series will feature the likes of Sango, Obatala, Oya...
The Dark Prophecy is much better than the first book. It's fast paced and full of humour and sarcasm. Lester/Apollo seems to be less narcissistic as he gets used to his temporary mortal body.
I'd say that Trials of Apollo is a lighter version of Heroes of Olympus (which at some point seemed really grim).