I also really enjoy how finding out about Greek mythology and demigods changes Carter's world. It's one of those stories where it really doesn't change the outcome of either series, but it's a wonderful idea to play with.
While the story was short, there's plenty of action. It's fun seeing Percy and Carter's different strengths I also liked the way he involved the regular kids in the fight . . . well sort of. The battle they have to fight isn't earth-shattering like it is when you get a whole novel (more like one of the bumps along the way), but it's still an important fight, even if it's not on a global scale. Plus there are hints of a greater issue going on that just makes the entire thing exciting. Honestly, I'd love to read another one, maybe even a series of novellas where Percy and Carter, or maybe Sadie and Annabeth work together.
My only complaint with the story is we only get it from Carter's POV. I would have liked to see what Percy thought of the whole situation and of Carter in particular. I don't know if they ever will actually make a series when all of these different characters come together, but it would be fun.
All in all, this is exactly what I'm looking for in a novella. It's fun, action packed, and absolutely unputdownable. If you want a quick fun read to hold you over until the House of Hades comes out in October, you need to check this out. It's an amazing 4 stars for me and a must read for all Rick Riordan fans. I'm just hoping there are more in the future.