In short: Though Philippa Gregory is well known and liked for her adult historical novels, I do not believe her writing and characterization translated well to her YA debut, Changeling. 
 The synopsis for Changeling is a bit misleading. I was led to believe it would be historical fantasy, what with the mention of werewolves and witches, but instead it is just plain historical fiction. Not that that's a bad thing - after all, historical novels are what author Philippa Gregory excels at. Changeling is set during the Crusades, a period of utter ridiculousness, which allowed for a plot that was ripe with possibilities for entertainment. Seriously, the stuff people used to believe - without any concrete evidence beforehand - is beyond insane and unfounded. Philippa Gregory makes sure to exploit the ridiculousness of the Crusades, with her expert knowledge of history, to its full extent. 
 Despite this, I can't say I was completely entertained and enthralled with Changeling. The story felt very disjointed as it was split into two completely separate story lines, one revolving around a nunnery and one a werewolf, and neither of them were particularly interesting. The characterization was also bland, save for a few secondary characters, making it hard to care much about what happened to these characters. And the writing, to be honest, left much to be desired, which was surprising because I was expecting a lot from Philippa Gregory. But at best the writing was bare-boned, without any flourish or interest, and at worst, kind of... well, bad. (Note: I really dislike describing an author's writing as bad because I am by no means a good writer myself, obviously, so my apologies for my hypocrisy here). 
 I have not read any of Gregory's adult historical novels, but I understand from their popularity that they are quite well done and detailed. It's unfortunate then that my first exposure to her work was though Changeling, her YA debut, as I do not believe Gregory's writing and characterization translated well to YA. As such, I will not be continuing with this series. It wouldn't feel fair for me to judge her writing just based off of Changeling though, so I would like to give her adult works a try sometime.