I start a new book for each game when I start the restoration. I record the dates I started, finished, took to shows, and high scores inside the front cover. After that I use it as a list of "needs" on that particular machine, as small things/problems pop up like flickering bulbs and other adjustments I might want to make. Then, when I do actually get around to addressing these needs, I record the fixes I did and the date. This way I can look back and see if it seems like I've fixed the same problem more than once.
I don't leave it in the game when I sell it though...usually the new owner doesn't want it, but I usually do offer it. I use these small memo books - I bought about 800 in a mega-pack somewhere along the way...