For just about every book I've written, I've kept a large looseleaf notebook filled with notes, correspondence, photocopied medical studies, etc. If I kept all these notebooks on my shelves, though, they'd crowd me out of my office eventually-- so I reuse them. I have two big ones (1 1/2 inch ring binders) and two smaller ones that I keep swapping out when it's time for a new book.
It's always a strange feeling, though, tossing out the old. Each of those notebooks represents several months of my life. But when I look through them, I realize I'm done-- there's no need to hold on to a hundred newspaper articles about bullying when the book is already in bookstores. So I throw the notebook contents in the recycling bin and put a new label on the spine of the notebook... but it's still hard to do.
Only once did I regret it, and that was my Celine notebook. I still know which one it was. I replaced it with notes for another book, but every time I look at it, I think, "That's my Celine book. That's the green binder I lugged around to Vegas, the one I carried with me through the casinos and shows and hotels. The one I read and reread in the hotel hallway at 5 in the morning so I wouldn't wake anyone." I tried to distance myself from it and pretend it was just another job and that it was time to move on just the same as I always do, but I was faking it. That was an experience.
Maybe this next book will be, too. If so, I promise to let myself keep the notebook in tact for sentimental reasons.