Does not follow the real book at all and even makes up some random stuff as well(like how they didn't actually go to Nashville). They didn't even get the greek myths correct, let alone the book. Why is Persephone in the underworld in the summer. She's supposed to be up on land with Demeter. I understand why they didn't mention Kronus or anything, probably because they think that there won't be another movie, but then don't title it Percy Jackson & the Olympians: The Lightning Thief because it does not follow the book at all
If you're gonna make a movie and put a bestseller's book as the title, at least get the whole thing accurate. And don't say that the movie would be too long, this is what you get if you want to do something like this