The Red Pyramid was recommended to me by my nine-year-old grand niece. She loved the book. I didn't love it. I wanted to, but, it was too flawed.
The book features two siblings, Sadie, 12, and Carter, 14. Unfortunately, they are so cliched as to be cardboard caricatures. Each chapter is told in the voice of one of these two - and while the reader is told who is narrating, if you don't look, you may not be able to tell the difference from one chapter to the next. Sadie and Carter almost have no personalities. Carter is subdued and grew up traveling the world with his Egyptologist father while Sadie is brash and grew up in the care of their grandparents. Riordan apparently believes that those facts are sufficient to provide the reader all they need to know about his narrators.
I think many young (grades 4-9) readers will love this book. There's lots of action and both good and bad characters. Along the way, they will learn about ancient Egypt.
My grandniece, who read the book a couple of years ago, liked the book a lot.