in all of the books I go back to - human observation.
This is pretty true for me, too.
I read from the beginning to learn about people, and have enlarged that to reading re people in places at various times. I can be caught up, kind of stunned, by a turn of phrase, some sentences that seem to me to encapsulate an insight or simply describe something very well.
I do read words rather like a bath of pearls, or, I suppose, tripping swine, and tend not to process data to deep memory, except by continued exposure to more about the data over time. I end up with a vaporous overview of history, but with some almost random pictures of how things were, as if I were there.
I'm presently rereading Barbarian in the Garden, a book of essays by the Polish poet Sbigniew Herbert (member of resistance, trained in law, economics, philosophy, literature, obviously knowledgeable about the history of architecture and art). I first read it twenty years ago, read part of it again maybe twelve years ago. The essays are about Lascaux, Paestum, Arles, Orvieto, Siena, Gothic Cathedrals, Albigensians/Inquisitors/Troubadours, the Templars, Valois, Piero della Francesca. It's still a keeper.
I'm a maven of travel diaries and thus have read - besides Herbert - Montaigne, Goethe, Dickens, Twain, and many more diaries, most of those prime for a re-read.
On the other hand, I rarely reread a mystery or police procedural except by mistake. I can imagine rereading William Trevor and Alice Munro and some other story writers.. though I haven't yet.