I was completely engaged with this book, but wonder about it's designation as a YA title. There are two reasons for my concern--the content of the story and some of the features of the writing.
This wouldn't necessarily be an easy read for young adults. Perma-bound recommends it for 5th-9th grades with a 570 Lexile estimate. The plot begins with Melkora's (the Irish Princess), brother having his hand inexplicable cut off while shopping in Dublin. Shortly after that Melkora and her younger sister are stolen by slavers. The remainder of the book is about her long voyage as a slave. Rape, physical abuse, and the lives of slaves are the backbone of the remainder of the plot. I certainly wouldn't want to have a 5th grader reading this material.
The writing is also a bit of a challenge. Napoli makes some leaps during the narrative that might well confuse less mature readers. Occasionally there are jumps in time during the narration that aren't obvious. They would be confusing to readers. Readers might also be confused by the historical period. I don't think many 5th-9th graders are familiar with Europe during the Dark Ages. Napoli is not specific about her time frame, but it is during early Christianity but post-Roman Empire. I believe the setting/tone/events of the book will provide a relatively accurate picture of the time period. Certainly much more so than some of the popular movies right now. The characters become relatively well developed through the book. The ending was not tidy--many readers will finish feeling dissatisfied.
This would be a useful companion reading for a high school world history course. I would, however, strongly advise students that there are some very "hard to read" parts of the book (based on content). The rapes are not sexually explicit, but they are quite clearly rape. Not all students or their families would find the content acceptable.
All that said--I am still haunted by the book. I'm certainly not an historical scholar, but I believe that the rich portrayal of this time period in Europe and the Middle East is worth the challenging materials in the book. It would certainly liven up the study of the Dark Ages!