This book is one of my childhood favorites, and although it's aimed at a younger audience I still occasionally re-read it as an adult.
I found the title a little silly even when I first read it at age eleven, but the unusual name (Wren) drew me to at least read the summary on the back, which lead me to read the first page... and then I was hooked. Wren is a cheerful, energetic protagonist who immediately captured my heart, I quickly grew fond of the other youthful protagonists, and I loved reading their adventures.
However, what really made this book memorable for me was a supporting cast member who was both an adult and morally ambiguous -- even though these were both character traits I found hard to respect as a child.
At age eleven, I had never really questioned why villains did bad things, assuming they did evil for the sake of doing evil. It took this key adult character relating her childhood (her family did not speak of morality, but simply of power, and what it could accomplish) to get me questioning the characterization and motivation of antagonists. (Somewhat ironically, the Evil Emperor in this book comes off as a pretty standard "evil because he wants to be" villain.)
This character is not the only adult character I found likable in the Wren series. Unlike the adults in so many childrens' books that feature clueless and/or boring adults, the adult characters in this book are written as competent, complex people whose interactions with the child protagonists are marked with understanding and attention rather than condescension.
So I remember this book not only because it was fun and exciting, but because it quietly broke from the cliches I had grown used to, and because I learned from it.