While it isn't necessary to read Riordan's Percy Jackson and the Olympians series prior to this novel, The Lost Hero takes place afterwards so the reader will be missing some background. Since this novel focuses on three characters that are new to this world, however, it is very easy to just follow and learn along with them, while realizing that there are past relationships and nuances that you aren't in on. The Percy Jackson series is fun, and there are only five, so they are definitely worth checking out and this book contains minor spoilers for that series, at least regarding whether certain characters lived or died (Riordan does a good job of not giving away much about the big events and twists, though).
The novel begins when Jason, one of the three new heroes, wakes on a bus, surrounded by strangers, unable to remember anything about himself. The first two students to address him are Piper, who says she is his girlfriend, and Leo, apparently his best friend. After an attack from a wind spirit, they are rescued, and introduced to a whole new supernatural world, one in which gods truly exist, and they are demigods, the offspring of a human and a Greek god. Piper and Leo are quickly claimed by their supernatural parents, though Jason isn't. However, as confused as he is, Jason appears to have some familiarity with what is going on, recognizing the gods, the symbols, and much of the mythology, even if he keeps using the Roman rather than the Greek terms. All three are under a spell or mist - Piper and Leo have been made to believe that they know Jason while Jason's memories have been taken. Soon after their arrival, they are given a quest to free Hera from her captors. In the process, they hope to find out what is going on with Jason and why, and Piper has her own motivations as well.
Meanwhile, Annabeth is attempting to find Percy who has gone missing. A prophecy told her that Jason was a clue of sorts, so though she continues on her mission, she hopes this quest will lead to answers about Percy.
I thought this was a very fun read, and even though it takes the heroes awhile to put two and two together regarding Jason's background and mysterious appearance, I enjoyed it. There are various hints about what may be going on, and the teachers appear to know even if they are forbidden from talking about it. I was actually surprised Annabeth didn't figure it out much sooner, but it will definitely be fun to see where the story goes from here. I also enjoyed the ways that Riordan used this novel to explain the differences between Greek and Roman gods early on, when even Annabeth, Camp Halfblood resident genius, thought they were the same with new names.
I can't wait to read the rest of the series, and Riordan is rather reliable as far as publishing goes. I just wish the paperbacks came out more quickly (maybe it's just me, but if the sequel is out in hardback, that means all preceding novels should be available as paperbacks). I also wish the Percy Jackson movie had been more successful (and better made from what I hear) because this could have been such a fun series to watch onscreen, especially since each novel covers one quest rather than one school year like the Potter series.