The most obvious difference between the book and the movie is that there is no mention of the god of war, Ares.In the book, Percy and his friends help to get Ares shield back from Waterland and in return, Ares provides them transport to get to Las Vegas. However in the movie, Percy, Annabeth and Grover used a truck parked outside Medusa's lair(Aunt Em's Eating Eurporium)to get to Las Vegas.
the second difference is that in the movie, Chiron and Grover knew all along that Percy was the son of Posidon, the god of the sea.In the book, percy only discovers this when he was attacked during the capture the flag game by a hell hound.
The third difference is that there the oracle(the spirit of Delphi)does not appear in the movie to tell him about what would happen during his quest to the Underworld. Instead, Hades, the god of the Underworld appears from the camp fire after the capture the flag game and forces percy to give him the lightning bolt in exchange for his mother, who was kidnapped by a minotar when Percy and Grover were entering Camp Half-Blood.
The forth difference is that in the movie, one item was stolen, which was Zues'(god of the sky)lightning bolt, which gave Zues power. In the book however, two item were stolen, the lightning bolt and Hades' helmet of Darkness, which allowed Hades to hide in the shadows and become invisible.
The fith difference was where the item(s) were hidden to fool Percy thinking that they were not with him. In the movie, the lightning Bolt was hidden in the shield that Luke gave Percy and it would only appear when Percy was in the Underworld. In the book, the helmet of darkness and the lightning bolt was hidden in the bag that Ares gave Percy, but the items would also only appear when percy was in the Underworld.
The sixth difference was where Luke battled Percy. In the book, Luke fought Percy after he completed his quest at Camp Half-Blood. However, in the movie, Luke fought Percy at the top of the Empire state building when Percy was going to return the lightning bolt.
The seventh difference is that there is nothing said about Thalia's tree(Zues' Daughter that got turned into a tree by him in order to save her from dying)in the movie.
The eighth difference is that there is no mention of the god of wine, Dionysus.
The ninth difference is that Clairese(daughter of Ares and Percy's arch enemy)does not appear in the movie and there is nothing said about her.
The tenth difference is that in the movie , percy and his friends have to go on a quest to retrieve three pearls that would help tem get out of the Underworld. In the book, Percy receives the pearls from a Nereid(spirit of the sea).
The eleventh difference is that Kronos(god of time)does not appear in the movie and it was Hades plan to steal the bolt from Zues through Luke(gods cannot steal each others items).In the book, it was Kronos' plan to get the bolt and lead Percy Jackson to his doom.
The twelth difference was that in the book, Mrs Dodds, a Fury(servent of Hades)is killed by Percy when Chiron throws him pen which turns into a sword. But in the movie, the fury escapes when Chiron arrives.
the thirtenth difference how Percy's sword(Riptide)is activated. In the movie, Percy clicks the pen and it immediately transforms into a sword made of celestrail bronze. however in the book, Riptide is uncapped in order for it to turn into a blade.
The fourteenth difference is that Percy did not have any dreams about his quest throughout the entire movie unlike the book, which Percy dreamt about his quest in his sleep.
between the book and the movie, I would say that the book has won the battle over the movie as it give a very vivd description of the characters and is more action-packed than the movie.
