The Mark of Athena is my new favorite book by Rick Riordan. I loved it. It ended (as always) on a real cliffhanger. I have to admit, I didn't expect that at all.
I love how we knew who Annabeth was going to face on her journey alone because it allowed us to try and come up with ways she would succeed. And then she didn't do it at all how I thought she might. I was expecting a fight to the death or something to that effect, but (of course) I was wrong.
Riordan brought forth a lot of new myths he hasn't used yet and introduced us to what I think, are people we didn't expect to meet. Plus, we got to see the Roman side of Mr. D. and learn a little bit more about how the whole Roman and Greek thing was affecting the Olympians.
I have to say though, one of the most intriguing things for me was his explanation and revelations about Greece and Rome. It really is an interesting plot and I have no idea how Riordan can get any better than he already is. But, I know he will because that's how he is. Each book gets better and better (I say this with the exception of The Sea of Monsters; I didn't like that one as much).
Now, just to sum it up. Excellent book. Read it and love it!