I'm reviewing the Balefire series as a whole because, while it is currently out of print, it will soon be reprinted in one volume some time around the holidays. However, I'll try not to give too much away in case you want to hunt down the original volumes (A Chalice of Wind, A Circle of Ashes, A Feather of Stone, and A Necklace of Water).
Clio Martin and Thais Allard are twins, separated at birth by their grandmother, Petra, to protect them from the questionable machinations of people within her own coven. Thais goes with her father to live in Connecticut, while Clio remains with Petra in New Orleans. But when they are 16, their father is killed in a mysterious accident, and Thais finds herself in the hands of strangers in a city she never thought she had any connection to. Running into their twin the first day of school is the least of the surprises the girls face as they both discover their family's true history, one that goes back further than either of them ever imagined.
It is a mystery that these books ever fell out of print, and it is a blessing that they will be brought back. Clio and Thais's world is vividly drawn. Tiernan's New Orleans is a character in itself, emanating from the page. The Balefire coven is filled with rich, complex characters, from the duplicitous Luc-Andre, the penitent Marcel, the haughty Axelle, the wise Petra, and the fiery Richard. The system of magic in this series is so organic to the world and very believable.
One complaint is the somewhat rushed ending -- Tiernan had planned a 15 book series, but low sales required it to end at four. However, she has tweeted her intentions to smooth it out for the reprint. Also, it switches perspective between Clio and Thais every other chapter, which I don't mind, but I know that annoys some people. Overall: