Book Eleven (I think) of the "Sky and Companions" series.
More then a year has passed since Sky came home from her previous adventure, when Sky gets a letter. Upon being told she cannot leave for another adventure by both her foster parents and the mayor, and angry Sky, steals the mayor's secret talking horse and escapes into a tunnel. The problem is, that the horse is more in control then her and they are heading to an island of cannibalistic Island Eaters.
Sky as an Adventurer.
Sky before she started adventuring.