I used to finish a book and forget I'd read it almost at once. Not the feeling. The feeling lingers, in odd places, surfacing years later when I'm doing something unrelated. It's the facts that go. The title first, then the author, then the shape of the plot softening into something vague and pleasant, like a dream I can't quite get back.
It took me an embarrassingly long time to start writing them down. I already kept a notebook for stray lines and useless thoughts, and I kept, separately, nothing at all for the books, and it never once occurred to me to fix that until I was well into my thirties.
It began as one note on my phone. A title, a date, sometimes a line. Loved this. Struggled with the middle. Cried on the bus, which was mortifying. Over a few months it turned into a habit, and habits have a way of teaching you things about yourself you weren't looking to learn.
The obvious thing it does is hold the facts in place. I can look back at February and see that I read three novels and an essay collection and gave up on a book about productivity halfway through. I count the abandoned ones. Leaving them off felt like lying, and besides, the abandoned ones are sometimes the most honest entries I have.
The less obvious thing is what it does to the reading itself. Knowing I'll write something down at the end makes me read a little more awake. Not more critically. More present. I notice the sentence I'll want to keep. I sit with an ending instead of closing the book and reaching straight for the phone. I ask myself whether I actually liked the thing, which turns out to be a surprisingly easy question to never ask.
I don't give books numbers. A score flattens something I'd rather keep lumpy. A book can be clumsily written and matter enormously. A book can be perfect on every page and leave me cold, and a number can't hold both of those at the same time, so I don't ask it to. I write a sentence or two instead, and whether I'd press it on anyone, and which anyone.
The real gift is going back. Reading what I thought about a book two years ago and either nodding at her or wanting to argue. Seeing the months I read greedily and the months the list just stops, which I always remember as having had reasons. A reading life turns up on the page slowly, in small marks, and you can't see the picture in it until there are a few years to look at.
If you don't keep one, I'd say start, gently. It doesn't need to be tidy. It doesn't need to make sense to anyone but you. That's most of the appeal, that it's allowed to be yours and nobody's business.