Grayling's Song by Karen Cushman

Hannah Strong is a wise woman and healer.  People from the countryside and nearby villages travel to her and Grayling's cottage for potions and salves.  Grayling spends most days gathering herbs, plants and other items from the earth so Hannah Strong can make her remedies.  Grayling often feels insignificant.
Then an evil force sweeps through the country.  The force removes the grimoires (spell books/potion books) from magic folk and roots the magic folk to the ground.  Slowly the magic folk are turning into trees.  As Hannah Strong is rooted to the ground she commands Grayling to find the remaining magic folk, find her grimoire and reverse the evil magic.
Grayling lacks confidence but as she continues in her journey and befriends interesting individuals, she grows in wisdom and courage.
Warning: magic, an evil force, witches
-Borrowed from the library.