The Blood of Olympus: The Heroes of Olympus, Book 5

The epic battle for the security of the universe finally ends. Gaea has brought out her giants and has managed to get the blood needed to appear. The demigods have managed to get a statue of Athena in position to help them. However, it is not enough. There is also an imminent battle between Roman and Greek demigods. It is narrowly avoided. The leader of the attacking Romans ends up launching the crippling blow, which ends up being suicidal also. Leo also goes on a near-suicide mission with his mechanical dragon. His dragon gives him the "resurrection" position and they are able to meet with Calypso on the hidden island. The world is saved and the Greeks and Romans now live happily together... but Apollo is in trouble. There are new scenarios, but it is the same basic Percy Jackson that kids like.

The House of Hades: The Heroes of Olympus, Book 4

The epic "kids save the world" quest continues. There is a quest through the underworld that culminates in an elevator ride up. Leo ends up on an island and falls for Calypso. There are ghosts of Roman legions that come out and fight against bad demons. There is a mind-cleared "Bob the Titan" who comes in at various times to help the kids. (Memory erasure seems to be a common theme in this series.) We also find out that the gruff satyr coach is married. The Romans and the Greeks are somewhat working together. There is plenty of rolicking adventure, though the book is longer than others.

The Mark of Athena: The Heroes of Olympus, Book 3

The Greek and Roman demigods are together. They (barely) manage not to kill each other. They also survive competing love interests. They need to go on quests. One takes them to an aquarium in Atlanta holding a lot of mythical creatures. They barely manage to survive (even as one turns into a goldfish to save himself.) There is plenty of adventure and comedic relief as they discover more about what is going on, the weird condition of the gods and the differences between similar Greek and Roman deities. 

The Heroes of Olympus: The Demigod Diaries

The Heroes of Olympus: The Demigod Diaries by Rick Riordan

It is nice to be able to enjoy some Percy Jackson books where the stakes are not so high. Sure, there is the risk of death from monsters. But, "forgetting kiss anniversary" is more important than saving the world. The stories range among a number of topics. Some provide backstory for characters in the novels. My favorite was just a fun romp. Hermes asked Percy to retrieve his staff. They battle a giant to get it back. However, the giant is not familiar with how to use it and is just as likely to send out credit card receipts as he is to use a laser gun. The snakes also provide some comic relief. And in the end Percy resolves the "dating" issue with Annabeth. 

The Son of Neptune (Heroes of Olympus, Book 2)

The first book in the Heroes series had Jason with the Greeks. This one has Percy Jackson hanging out with the Romans in San Francisco. He has some memory loss, but it was not as bad as Jason's in the first book. He teams with some Roman demigods (including a reluctant son of Mars) to go on a quest. They are treated as relative outcasts by the Romans who are already into their bureaucratic ways. They manage some trickery of luring a giant out of Alaska so as to defeat him outside his home turf. Like usual, there is plenty of action and end-of-the-world situations.

The Lost Hero

Jason wakes up and finds himself in a camp with a "girlfriend" he has never seen before. His has no memory, but there are plenty of people that want to kill him. He realizes he (along with his "girlfriend" and one other student) are half-blood descendent of a Greek god. However, the gods are not talking with people these days. They make it to the half-blood camp. Unfortunately, there are others that are trying to destroy the world. He gradually finds out a little bit more about his true identity. He is a "Roman" half-blood on an "exchange" to Greek land. (Percy Jackson is on a similar reverse exchange.) He actually does like the "girlfriend" Piper, though discovers that she is the daughter of Aphrodite and a famous action star. They go on a quest, battle a bunch of bad guys and finally reach a conclusion, setting up things for the next book.