I started my records with the idea that what I am reading reflects my interests at the time. I'm not so sure of that now, but I think it probably does map my growth as a reader and I can also see authors that I have returned to again and again.
I started keeping track on paper in the summer of 1992 because I knew I was reading a lot, and wanted to know how much. A couple of years back I converted all my lists to Excel. I still keep the primary lists on paper, and convert them over periodically.
Mostly I keep track of the basics, including number of pages (to keep a running average), and I recently added year first published (for my annual "best of" list that only includes books first published in that year). I only keep track of books I've finished, no matter print or audio (though my wife claims that listening is not "reading"), and regardless of their source.
You bring up an excellent point and one that I have thought of making into a separate discussion. Is listening to a book the equivalent of reading it? I keep track of my unabridged audio books on my list. I do note them as "books on tape", the old fashioned library term for them, but that is for the purpose of locating them later if I wish.
If you are listening to an unabridged version of the novel, then it usually takes much longer than reading 'with the eyes'. I think you also have a chance to dwell more on the impact of the text. So yes I count them into my reading.
I have discovered one of the drawbacks of my database list is that I haven't recorded a book's ISBN so I have been unable to import my lists into an online system like Library Thing which uses the ISBN as a unique identifier.
I agree that listening to an unabridged audio is slower than reading. There are times when I am one hour or so from the end of an audio, so I grab the book and finish it in 20 minutes or so. I think I am an auditory learner because I tend to retain information that I hear. If I both listen to and read a book, then I know that book cold. And yes, I will do that. I love to listen to my favorite books. I also use the audio if I need to re-read a book for a book club.
When I started my list, I didn't know what an ISBN was, so like you I don't have ISBNs in my list.
I seem to have a photographic memory when it comes to books ive read, but to very little else. Its rare i will look at a book and not be sure if i have read it, so i suppose i keep a record of a sort, in my head, LOL.