So in The Mark of Athena we have seven demigods working together to find the Mark of Athena and try to stop Gaea. Always trying to stop Gaea. I just love these kids (demigods) so much! They're fun and funny and never-say-die and just so real. I don't really have too much to add to this review over my review of The Son of Neptune. It's all the same characters from The Lost Hero and The Son of Neptune but all mashed together on a flying (literally!) ship on a mission. They all work well together.
I think the best way to describe this is as though you took all the very best parts of the first two books and put them together in one book and then took all the characters a little deeper. I'm 110% looking forward to the next book, House of Hades.