One of the main factors that makes me like a book (or not) is whether I like the main character(s). I enjoyed getting to "hang out with" and get to know the main character, Maris, for the several crucial months of her life that this book covers. I appreciated her longing to not be pushed around by all the other people in her life (her teachers, parents, roommate, and boyfriend) and to do what's right for her. I could relate to her struggle to learn who she is, and (since I like history myself) I enjoyed her fascination with Roland and with St. James despite the pressure of more modern views.
A main theme in this book is how people tend to try to make other people into who they think they should be. Maris has to resist her male peers' pressure to give in to their desires. As she journeys, she realizes that, likewise, people thru the centuries have cast St. James according to their own desires. My chief disappointment with this book is that, at the end, Maris does not seek after the St. James who followed Jesus Christ; I wondered whether Maris had chosen to believe St. James was who she needed him to be. I also wondered what happened to Maris after the book closes (though this is probably a sign of how engaging the book is rather than a flaw in the plot). Though I am a Protestant of a fairly conservative stripe, I still appreciated the author's portrayal of how a religiously liberal young person becomes an adult; I found it illuminating. I agree that, through the ages, Christians have done a number of non-Christian actions in the name of Christ or the saints.
In summary, I enjoyed the book. I wish its portrayal of France and Spain had been more vivid (though the book has great photographs as illustrations), but maybe -- who knows? -- if I ever go to Santiago de Compostela myself, I will remember this book and appreciate it more.