I keep most of my receipts in a folder, and use Quick-Books to keep track of my money (or lack of) It has a place for each of my projects and could total them if I ever wanted to know (haven't yet).
What I'd really like to find is something to keep track of part numbers and components I use on the truck. Because very little is stock it could be difficult getting replacement parts at the auto parts store. Not that they won't have it, just I have to remember what to ask for from what year and vehicle.
A blank parts manual would be good, it would have all the common listings for parts and a place to fill in the part number and alternate part numbers if any. then as I buy kingpin sets, wheel bearings, rotors, master cylinder, spring pins, U-joints, lug nuts, calipers, pads, tie rod ends... all that stuff would be easy to look up and order again when needed.
Grigg