The Tapestry at Briarmount Abbey. The Book of the Holt

The Tapestry at Briarmount Abbey is a Book of the Holt novella, a story set several years before The Dragons of Deepwood Fen, and focuses on Rylan Holbrooke, one of the heroes of the main series. Rylan Holbrooke, dragon singer and part-time thief, travels to faraway Briarmount Abbey only to learn that the Sylvan Tapestry, the very thing he'd come to see, was stolen mere hours before his arrival. When signs point to the thieves having used magic to abscond with the ancient relic, it puts Rylan on edge-the last thing he wants to do on his holiday is tangle with a rogue witch-but he considers the matter too important and offers to search for the tapestry. With the help of Vedron, his acid-spitting dragon, Rylan sets off to find clues.
The trail leads him to the hills known as the Winding, which are said go be haunted by the Dancing Willow. The willow is home to a band of undead children who pray on the unwary or those foolish enough to remain in the Winding for too long. Rylan digs deeper and discovers the theft is related to the willow. In fact, the theft's origins can be traced all the way back to Black Aerlath, the terrible day when the willow was made and the children were turned into haunting specters.
The children didn't deserve their fate, and Rylan is desperate to help them, but it seems as though the local constabulary, the willow, even the abbey itself, are all working against him.