Rick Riordan's The Last Olympian is the last of the five books in the Percy Jackson and the Olympians series. These are some of the themes it explores:
The topic of family runs through all the book in the series, as all demigods have to make sense of their relationships with their godly parents. Percy mentions several times that because all gods are related to each other, all demigods are one big extended family. When Luke dies, they mourn him as a brother—even though he betrayed them by joining Kronos: Percy says that his voice is breaking when he asks for "a shroud for the son of Hermes."
Demigods have to rely on each other and trust each other in order to defeat the forces that are much bigger than they are. Percy and Annabeth are loyal to each other and trust each other implicitly, even though they have very different ways of doing things. Percy discloses to Annabeth where his "Achilles' heel" is because "if you can't trust Annabeth, you can't trust anyone."
Throughout the book and the series, a handful of heroes has to fight armies of monsters. The demigods have to constantly undertake quests that seem impossible to accomplish. Without hope, none of them would survive. This theme is tied with the theme of the home and family when Percy tells Hestia, the titular last Olympian: "Hope survives best at hearth."
- Honor and dignity versus humility
Percy is not impressed with the ways some gods use—or abuse—their power. He develops an understanding that true honor is earned by one's actions, not status. In a rather astonishing act of humility, he rejects the honor of becoming a god. This, of course, also demonstrates his loyalty to his mortal friends, particularly Annabeth.
There is an inequality between the gods and their children: "minor" gods do not have seats on Olympus, and their children do not have cabins in Camp Half-Blood. They are more likely to turn against the gods because they feel excluded. The same is true for Hades and his children even though he is one of the Big Three. Percy points out that this is the cause for the internal discord that made the gods (and, thus, the civilization) vulnerable to Kronos. He suggests that this should change, as everyone is equally important in maintaining peace.