The Tower of Nero: The Trials of Apollo, Book 5

In the conclusion of the Apollo series, Apollo finally makes it back to the gods. There are lots of fights before we get there. Nero is a super strong megalomaniac who wants to burn New York to a crisp, but he has trouble finding the right remote control. The half bloods and troglodytes help work together to prevent it from happening. Nero's minions are driven by fear. Alas, that is not enough to maintain loyalty. (One "friend of Meg" helps them out.) There are many close calls, but all ends up well in the end.  

The Tyrant's Tomb: The Trials of Apollo Series, Book 4

Apollo completes more quests. He gets back some of his godly strength. They find the silent god and manage to make enough peace to survive. There are some epic battles. Some bad guys are destroyed. Fans of the Percy Jackson books will probably love it. It does seem to continue on with what is regularly delivered. Here they do branch out to more of the Roman gods (and even some "adopted" gods from other lands.)

The Dark Prophecy: The Trials of Apollo, Book 2

The second Apollo book continues his adventures. He is still stuck in mortal form. He finds new friends, and continues with some old friends. There are also various monsters as well as super expensive tator tots. He for the most part remains mortal. However, he is able to emit a shining burst of light to help defeat others. There are other periods where they "bluff" their way to victory.

The Trials of Apollo, Book One: The Hidden Oracle

Apollo wakes up in a pile of trash to discover he is a mortal. He gets mugged, but a girl (Meg) helps save him by throwing fruit. He ends up being bound to serve her is part of his trial. He decides to meet up with Percy Jackson to get to camp so that he can figure out what is going on. Things don't work out so well. Some immortal Roman empires are trying to take over all the oracles in their quest to dominate the world. Apollo and friends manage to put off the destruction (thanks in part to a late appearance by Percy Jackson). He does discover that girl has split loyalties. It is a nice spin off of the Percy Jackson series that clearly has more adventures coming. I don't think Riordan can write a book without the end of the world being at stake.