"A notebook is a repository of failed attempts and experiments, of thwarted intentions that may or may not turn into unexpected successes ... Who knows what will prove a dead-end and what the inspiration or material for a book?
Whatever size the notebook, it accumulates its value slowly, line by line and page by page as it fills with notes. Some sets of notes take on a life of their own. Others give them up for the greater good of a book, or a story, or an article. You never know what will prove useful or useless, which gives a notebook a strange and numinous importance. Something jotted down in two minutes might occupy you for the next three years... A whole notebook might potentially contain the rest of your writing life."
That's Lawrence Norfolk (he of the great John Saturnall's Feast) on notebooks - do read the rest of his piece here.