a logbook is issued by a sanctioning body like NASA when you want to race a car with them. It documents some particulars about the car including min weight and photographs. It is used by the tech stewards to record that your annual tech is completed, as a reference (e.g. min weight) when your car is being checked post-race, and to make notes about the compliance of the car at races so that the tech at the next race can ensure that you have corrected an issue.
For my own personal records during the build, I keep a multi-worksheet Excel spreadsheet and record notes, links, purchases, things to get and do, etc. -whenever I read things that I think I might be able to use later in the build. I have all the rules in it as well so it is easy to search and I make notes when I read on the forums how others are interpreting the rules and particularly how the series director interprets them!
Make sure you get a Bentley right away, and I recommend the Pelican Parts book 101 Performance Projects for Your BMW 3 Series 1982-2000.
The "Getting into Racing" book that was discussed a while ago is excellent also.
cheers,
bruce