I sometimes used to do what arturg does when I was reading paperbacks so I wouldn't be stuck on the subway with nothing to read. But now with ereaders that's never a worry, just jump to the next book.
I do always delete finished books from my ereader, and mark them in Calibre with the date finished next time I'm at my PC. After decades of reading, it can start to get fuzzy if I did read a book or not (although to date, I've always remembered correctly, but I want to be prepared for senility
).