Seeing "software for writing books", I can't help thinking of something where you'd give it a few details, then it would hourglass for a while and spit out something beginning "T'was a dark and stormy night"
Seriously, a quich search reveals some interesting items, such as:
http://www.novelist.ch/joomla/ Storybook, with free & Pro versions
http://www.storyhack.com/2009/12/07/...-texttree-1-3/ Getting a little old now, but looks an adequate outliner