i started keeping track last year, following a comment by someone here (it might have been you, elsi). my system is very basic but it's interesting to see how many books i read. but originally i was motivated to do it because i have a terrible memory for names and titles, and i remember some books i had read in the past and wanted to re-read, but i have no idea what the book is called (it was a library book, and i've not been able to find it at the library, despite searching).
i'm still looking for that book, actually.
hopefully now that i keep a list i won't ever have that problem again (and, as a bonus, i can accurately answer when someone asks me how many books i read in a year, which i would have been incapable of estimating on my own
). i should have started this years ago.