This was a really excellent book. On the Amazon scale, I'd give it four-and-a-half stars (unfortunate that Amazon doesn't permit half-star ratings to be given out)
First-person protagonist "Eff" (her first initial; her actual name is hardly ever mentioned in the book) is the eponymous Thirteenth Child, one supposedly born with cursed magic and destined to go bad according to many in her society. Her plight is made worse by the fact that she is the elder twin sister to a fourteenth child, her few minutes younger brother Lan -- who happens to be the seventh son of a seventh son, and thus destined for amazing power.
Just the interesting details of the magical systems that Patricia Wrede has worked out are fun enough to make the book worth reading, but in actuality that's just part of the novel. What really makes it WORK is that this is really a story of a pioneer society that just happens to be taking place on an alternate Earth, where somehow human beings never made the trip across the land bridge to North America, and the megafauna never quite died out... and there's functional magic, both for intelligent and nonintelligent creatures.
This is Little House on the Prairie (the novels, not the execrable TV series) crossed with the magical mystery/adventure of Lyndon Hardy's Five Magics/Six Magics series, with the hard-edged rational touch of Modesitt's Recluce series. Eff's family moves to the frontier and becomes part of a growing community at the edge of the "safe" zone behind a magical barrier, and Eff learns to confront her strange natural abilities and eventually master them, learning something about herself -- and everyone around her -- in the process.
One of the things I liked most about The Thirteenth Child is that while it certainly played to various tropes, some of the ones I find most annoying were, for the most part, averted. There's some sibling rivalry between Lan and Eff, but for the most part they're brother and sister united against the others. The parents are actually smart and supportive, and the one true jerk of the family gets his comeuppance and basically no one takes his part. It's shown that the whole "Thirteenth Child" schtick is, in fact, not even universal. Someone who could easily have been made a fairly stock rival/adversary becomes an ally of the family.
The book kept me not off-balance, but interested, to see which way it went, and it went in the direction I didn't expect about as often as it went in one I did. This is the delicate balance a good novel should walk; you want your reader to feel involved and reasonably informed (so he or she can predict a lot of what's going on) but not bored or overconfident that they know how things are going to turn out (so they want to keep reading). Ms. Wrede hit that balance perfectly in this book.
I enjoyed this immensely and have put the others in the series on my wish list!