How Do Authors Decide To Write A Trilogy?

Magic Lessons by Justine Larbalestier has an interesting premise. Her characters have magical abilities, but using those abilities shortens their lives. A lot. The young people in the book only have so much magic and when it's gone, so are they.* The logical thing to do would be to put a hold on the magic, but those who do end up going mad.

Talk about being damned if you do and damned if you don't.

Reason, the protagonist in Magic Lessons, comes from a long line of magical folks, some evil, some maybe not. She may be able to come up with a way to get around the die or go mad problem.

But not in this book.

Magic Lessons is the second part of Larbalestier's Magic or Madness trilogy, and it was a little hard to get into for someone who hadn't read the first book. I understand people loving serials, but I'm struggling with whether or not readers should know which "series" books are really serials. And if they are serials, should readers just not bother with books two or three if they haven't read book one? How does that benefit anyone?

Magic Lessons also seemed to drag a bit. It seemed to take a long time for things to happen, and characters often asked obvious questions, which also slowed things down. This second book just seemed to be a continuation of the one storyline about these young characters struggling to stay alive and sane. Unlike, say, The Underland Chronicles, there wasn't an adventure specific to this book. At least, not that I noticed.

I like the basic magic or madness premise, but I wonder--did this story need to be told over three books? Would one tightly written novel have done the job? At what point did Larbalestier decide to extend the story over three books and why?

Magic or Madness, the first book in this trilogy, is on the ballot for the "preliminary ballot" (?) for the Andre Norton Award for Young Adult Science Fiction and Fantasy given by the Science Fiction and Fantasy Writers of America.

*According to Robert M. Sapolsky in Why Zebras Don't Get Ulcers, around 1900 a German physiologist believed that the human body was made up of a lot of finite things (breaths and heartbeats, for instance) and when you used them up, you were used up, too. I read that the morning after I finished Magic Lessons. I love it when that kind of thing happens.