Daja's Book is the third of the Circle of Magic quartet. (Most writers write trilogies, Pierce writes quartets.) The first two were pretty good, and I really enjoyed them. But so far, Daja's Book is the strongest of the quartet. And please note that I say "so far," because I haven't read the last book yet.
In this quartet four children are thrown together. They are from all different walks of life, and are vastly different. You wouldn't expect them to become friends, but somehow they do.
Sandry -- Sandry is noble born, and has lost both of her parents to a plague that she was lucky to survive herself. She breaks social boundaries because she traveled a lot with her parents, and usually the nobles of the places they visited didn't want to expose their own children to her. This led her to play with common children a lot, and to not particularly care about rank. That's not to say that she can't act like a high born noble woman when she wants or needs to, though.
Tris -- Tris comes from a merchant family, and she will always be the daughter of merchants at heart. But for years she was shuffled from one relative to another: strange things kept happening around her and they thought she might be possessed. Tris herself doesn't understand why the winds blow and lightning strike when she became angry. And she was angry a lot, because no one wanted her. Eventually cast out by her own family, Tris may as well be an orphan.
Daja -- Daja is from a family of traders, and her own mother captained a trading ship. The traders in this world have their own culture and language. They separate people into two broad categories: traders, and non-traders, and the non-traders are inferior. But Daja's life changes abruptly when her ship goes down and she is the only survivor. As though it is not bad enough that she has lost her family, she is now shunned by other traders because they are afraid of catching her bad luck. Daja is forced to live among non-traders, and carry a staff which identifies her to all traders as one who carries the disease of bad luck.
Briar -- Briar doesn't know who his father is, and barely remembers his mother. He grew up on the streets, and shares the company of thieves. We don't learn much of his childhood, and only meet him when he is about to be sentenced to work on the docks because he has been found guilty of theft for the third -- and last -- time. At the last minute he is rescued by a man who thinks that he will be a good addition to Winding Circle Temple. Briar isn't sure what to make of this stranger, but is happy to go along if it gets him away from the docks.
Each of these children find their way to Winding Circle Temple, thanks to Niklaren Goldeye (Niko for short). Niko is a great mage, and his latest project has been to find youngsters who have magic so unusual that the youngsters themselves don't realize that it is magic. Over the course of this quest he has rescued Sandry from a plague stricken land, Tris from yet more people who don't want her, Daja when the traders cast her out, and of course Briar from hard labor on the docks.
At Winding Circle Temple the four children meet, and the fun begins.