"Something useful" here refers to an opinion on what the book in question is "about", or an interesting lesson I drew from it, or a coherent reason why I might recommend it to somebody else.
Here, for example, is what I wrote down for A House For Mr Biswas:
> Finished yesterday, and found this primarily a story about a man who never really knows himself, and as a result can never quite situate other people in relation to himself. Fits and starts of self-improvement, a sense of life about to begin that persists for far too long, shifting roles played according to setting, a series of humiliations for which books offer some kind of solace and hope for betterment -- yikes. Like Proust, Naipaul seems to like unpacking the (nonobvious, often directly contradicting the obvious) motivations behind actions, and like The God of Small Things, the picture that emerges is one in which nobody is all that good, though it mercifully lacks its clockwork-mechanism-of-misery aspect
I generally find that books leave me with ideas and sensations that tend to just dissipate if I don't write them down. This is a way of combating that.
