My friend keeps a hand written journal of everything she's read. She gets most of her books from the library and more than once accidentally bought something on Kindle and got partway in and realized she'd already read it. She's someone who's really a fan of fancy stationary and pens, so everything is color coded etc and she uses it as a reference to keep track of authors she does or does not like. I'm lazy and buy everything on Kindle so I can just look at my library.