My first introduction to Roberts came when I stumbled across one of her Seven Fabulous Wonders books at the book store. 'Fabulous' is definitely the right word for this series of books inspired by the seven wonders of the ancient world: a mix of adventure, fantasy, and historical fiction. Though the books haven't been published in the US, I've managed to read all but the most recent one (which came out in the UK in July).
The first volume of the Echorium Sequence, Song Quest is a wonderful adventure very much in Roberts' unique style. A coming-of-age story with both a male and female protagonist, the novel is full of fantasy, but also grounded.
Synopsis:If you like young adult fiction, check out Katherine Roberts.
Welcome to a world from another time -- where legendary half-creatures still exist. A world where nature itself can be controlled by unearthly music. A world where the forces of good and evil are held in harmony by the Singers who have mastered the secret Songs of Power. A world on the brink of destruction, threatened by a dark lord whose evil knows no bounds. Rialle and Kherron, two novice Singers, are all that's left to stand in the enemy's way. Stranded in a strange land with only one another to rely on, these former rivals must work together if they are to survive. In a timeless coming-of-age journey, Rialle and Kherron discover the strength of spirit that lies within them in their quest to help good triumph over evil.
You will not be disappointed!
Okay, I know that's a bit of a pronouncement given the fact that I haven't read all of her books, but I figure that if seven of them are good, there's a pretty good chance that the rest of them are too. ;)