Maybe it's because I'm more used to reading stuff like the Wheel of Time, but these books, although quite good, don't really seem to have a properly threaded plot. First book, good. The ending was properly set for book two. Then book 2, the villain, who is overwhelmingly powerful is defeated, almost accidentally. Then book 3, the evil half brother, who nobody really knew about, gets toasted by a uncontrolled experimental technique. But then in this book, number 4, he isn't dead after all and is intent on revenge for his mutilation. All this takes place over a good 18 months, so it's not like the good guys don't have time to talk to each other. I think the biggest hole has to be the twin sister. There's this big back-up plan if Tristan dies. He can't be trained because his blood is blue; fair enough. He can't wear the stone to read the Tome for the same reason. So train HER, let her read the Tome and then you actually have enough knowledge and a big magical ability to call on.
It just seems that the whole series is riddled with quick "By the way...." moments that then massively affect the course of events.