Consisting (so far) of Wren to the Rescue, Wren's Quest, and Wren's War. By Sherwood Smith.
I read these books for nostalgic reasons, having read them in my childhood (ha, ha) and remembering very little of them, until I stumbled on them again recently because Sherwood Smith is one of the authors in the Firebirds anthology.
Really, I don't think they're very good. The first book seems especially simplistic, depicting characters in a way that reminds me of a child's straightforward view of the world. No one is anything other than what they claim to be, two preteens can make a long journey on their own to the fortress of evil, etc. I admit I felt rather cynical when I assumed someone was a traitor (there were even hints that it was so) and it turned out not to be so.
The second and third books were somewhat better than the first, but still not great. There are certain obvious objections to raise, such as... if magical resources are limited, as Wren is taught, why does she never have trouble doing magic? The principle is mentioned once and then seemingly forgotten. It seems as if what she can do depends on where the author wanted the plot to go, not what she was actually capable of, since she seemed to actually regress after the first book.
Wren is an appealing character, with a greater destiny that's only hinted at in these three books, but I don't think her story is very well told.