UK Cover version of the Mark of Athena book.
Rick Riordan was amazingly gracious enough to answer a boat load of questions from fans via Twitter. Here are the ones regarding the books.
A: Yes. 
A: A true cliffhanger means characters end the story in imminent danger. Most of my endings aren’t actually cliffhangers. They just leave unanswered questions. Mark of Athena, however, ah… never mind.
A: The giant Alcyoneus is unique. He is only immortal in the land where he spawned, in this case, Alaska. Once outside, he’s fair game.
A: Nico has a key role in MoA but demigods are rarely safe!
A: Don’t think it’s too much of a spoler to say yes.
A: If I did, that would be way down the line. Heroes & Norse series first.
A: That’s true in PJO too. Many demigods do, but not all.
A: No spoilers. Sorry! But it will be explained in MoA
A: I don’t know. I’ve never had a reason to determine it, at least not yet.
A: She will make an appearance later in the Heroes series. Right now, that’s all I can say
A: Gleeson Hedge. He cracks me up. Also Leo.
A: Like I said, that’s waaaay far in the future, but some day, I’d like to do 1 more CHB series where all the action actually happens at camp, b/c most of the other books happen on quests. That’s part of the story that hasn’t been told, & would be fun. But like I said, that’s way, way in the future, so don’t hold your breath.
A: Yeah. Mrs. Dodds is a real math teacher, but she’s not actually a monster. Mr. Brunner (aka Chiron) is a real Latin teacher. Travis & Connor Stoll, Beckendorf, Miranda & Nico are named for former students of mine.
A: I can confirm that you shouldn’t listen to rumors.
A: Grover or Tyson. They’re so fun to write about.
A: Only because the first draft is so bad and my deadline is so close. No, actually I got teary-eyed when I wrote one scene in Mark of Athena, and no, I can’t tell you what it was. Yes, I know that’s mean.
A: Actually that question will be addressed in Mark of Athena.
A: The Argo II. Oh . . . not that kind of ship?
A: At the risk of hurting Blackjack’s feelings, Festus for sure.
A: different publishers make diff. covers. The thinking is UK kids r attracted to different cover art than US kids. *shrug* Each publisher has final say over the cover in that country.
You’ll notice that the humor in his writing only comes naturally due to his humor in some of his answers. You’ll be able to see more of it in the rest of his questions, which you can read on his blogsite here.
