As an adult, all of these are quick, cute reads with great cover art! The illustrations for each are good, too. The world-building is done in such a way to indicate these are books in a series. I was surprised to find out it's more than four, and plan to read only the first four. Ten years old is so young to rule, but I was around the same age when I first read this and didn't see a problem. The choice to name the girls with the first initial of their gemstone was clever. The creatures and peoples are introduced in way that further the plot, which is awesome. The villains are introduced effectively and remain obstacles throughout. This was fun.
Ruby Princess Runs Away--I was glad the princess was properly coronated, and she was still given room to act like a kid within reason.
Sapphire Princess Meets A Monster--It's established the girls do have advisors, which I hadn't thought, which is good.
Emerald Princess Plays a Trick--This was my favorite, favorite of the four books I read as a kid, from thee beautiful cover to my favorite gemstone to the interesting plot. And it remained interesting! The villains featured heavily, the world-building continued to be fascinating, and the ending was cute. There were a few more Disney references.
Diamond Princess Saves the Day--I liked the imagery in this, and how she talked about her sisters. The action was interesting, as was the plot-propelling prop. ( )