Riordan's first series, Percy Jackson and the Olympians, dealt with the demigod children of the Greek gods. In this newer series, we're introduced to the idea that those Greek gods and their Roman counterparts are just different aspects of the same beings. This first book introduces us to three new kids - Jason, Piper, and Leo - and the chapters in the book alternate among their points of view. We're also introduced to a lot of new supporting characters, from other campers in the various cabins at Camp Half-Blood (give it up for my brothers and sisters in the Hypnos cabin!) to various gods and other figures from Greek mythology - most of whom are damaged versions of their legendary selves. Riordan does a good job of teaching readers about the true mythological figures and stories, while entertaining us with new action and humorous situations.
My only real complaints are that the story felt a bit slow at times, and that the girlfriend/boyfriend stuff between Jason and Piper was unnecessary.