About the series:
Bujold is most famous for her science fiction books starring Miles Vorkosigan and his family, and for her "Five Gods" fantasy series (Curse of Chalion etc.). Her Sharing Knife series gets a lot less publicity which I think is unfortunate.
There are numerous online commentors which liken the series to Lord Of The Rings - but as someone who had trouble getting into LOTR, I can state that you will NOT have this problem with the Sharing Knife books. They're set in a very real landscape (along what we know as the Ohio and MIssissippi rivers, though not called that in the book) with civilization for the most part at early 19th century levels - and Bujold's prose is, as always, welcoming and easy to read. You get the sense that there are hidden depths to Lakewalker society, by the frequent reference to "maker's secrets" and a long-departed civilization's cities and bridges, but you're not choked with it the way Tolkein could sometimes do. As well, the "vanquish the ancient evil" here is a long-standing chore that is still ongoing as the series concludes, and the "change society forever" is shown as one person seeing a need, and gradually, in his own small (or not-so-small) way, beginning to effect that change to the benefit of everyone.
It too would qualify as fantasy, though in many ways it's much more accessible than the Five Gods books because there is not the constant dealing-with-deities undercurrent there. In many ways, you could consider this to be a historical adventure series, except there's this thing called "groundsense" and there are these awful creatures called "malices".
The downside of any individual book in this series is that while you could in theory read just one, really you need to read them in order as later events really don't make sense without the background from the earlier books. In fact, Beguilement and Legacy were originally a single book (split into two for length) as were Passage and Horizon.
Passage (the first book) deals with a Lakewalker (Dag) encountering a Farmer (Fawn), and starting the very beginnings of getting the two groups to interact more than they had in the past. It ends with the unthinkable: a Lakewalker and a Farmer marrying. Legacy dealt with Fawn's culture shock being plunged into Lakewalker society, the Lakewalkers' refusal to accept her, and Dag's beginning realization that a) his people don't know it all, b) are unwilling to learn, c) things have to change for everyone to survive in the long run, and d) he's the only one to realize that so he must learn how to change the world.
About this book (Passage):
In Passage, you see Dag beginning his mission of educating Farmers as to what Lakewalkers can do. It's the first time he or any Lakewalker has tried to bring down the veil of secrecy and teach Farmers what Lakewalkers are really all about, it's the first time most Farmers have ever seen Lakewalkers as allies versus terrifying, mysterious sorcerors, and it's the first time Lakewalkers (admittedly, just two of them) are taught to see Farmers as people worthy of respect. Dag also manages to solve one critical problem that has plagued the two groups (beguilement), opening the door for allowing Lakewalkers to provide healing to Farmers - and thereby opening the door for greater interaction between the two groups and greater protection against the malices in the future.
There is some violence in the climactic scene but nothing too bad. There is some blood-and-guts there (literally) but nothing too graphic. The only sex in the book is referred to obliquely - not even remotely graphic. It is by no means a children's book but I'd have no hesitation in letting a 12 year old read it.