Let's be honest, these novellas are for the fans. While they are a fun adventure in and of themselves, they're designed to be read by people who have read both the Percy Jackson books (at least the first series) and the Kane Chronicles. If you haven't, this is still a fun adventure, but you're missing about half of what makes it so great.
I loved seeing Percy and Carter work together in The Son of Sobek, so I was thrilled when I saw we get our own Annabeth and Sadie adventure. The Staff of Serapis didn't disappoint. I enjoyed seeing the two mythologies combined and the way Annabeth and Sadie relate to each other. It highlights how different the two characters are and their magic/mythology systems.
The adventure is, as always, quirky and fun, hinting at one more crossover book, at least. This series also helps tie up one small loose end from the Kane Chronicles, so you can tell the author's been planning this for a while. All in all, I'm giving this book a strong 4 stars. It's fun, full of adventure, and an excellent chance to see some of your favorite characters in new and different situations. So, if you've read at least one of Rick Riordan's series, you really need to pick this up.