My time cataloging made me think a lot about the books I've collected. The most interesting part of rare books is the physical evidence of previous owners: marginalia, inscriptions, things that remind you that the book you're holding is very old and was, at one point, well loved. It's cool to see the effect that books have on their readers, like little drawings of soldiers in a book about war, or marginalia that essentially warns future readers off from reading the rest of the book.
This is also the difficult aspect of cataloging. OTOH, these things feel tangibly important to the specific copy of the book, but these are also things that are difficult to quantify in terms of simple data terms. You have to sort of treat the data you catalog as markers for people who might give that marginalia a more detailed study. It also makes you treat your own books differently, more like a living record of yourself than like an object (I don't usually like writing in books).
Content-wise, I spent a lot of time thinking about frame narratives, the beginnings of novels that set up the rest of the story as occurring within another story. The first time I noticed this was in a novel called Wanderings of Warwick, that begins as a third person story about a genteel garden party only to become a first-person adventure story about pirates 20 pages in. After that, frames kept coming up. For some reason, straightforwardness is rarely a virtue for 18th century novels