I don't know how the rest of you keep track of your vehicle' but I have a pocket size notebook in my glove box, in that I have:
Amount of fuel, and it's total cost
Parts replaced and their cost
I started the log book out the the full price paid, all taxes and title fees, so I can figure out all kinds of numbers from that like out of pocket cost per mile, total fuel used in that car, total that I spent on parts, how buying a part will affect my average cost per mile, I found this book useful when I had to get my o2 sensor replaced under warenty, I didn't have a recept, but I could tell them the day that I bought it, and they agreed to replace it without argument.
If the book is permanently bound it will hold up in a coart of law. I created a few of them when I needed to make headings on the pages to keep things lined up - the xB book is not so neat however because of all the SG data I record as well . . . going to have to rewrite that book sometime soon.