I assume he means bookcases used AS bookcases, not shelves for snow-globes, Beanie Babies or Tourist Trophies.
I enjoy “reading” bookshelves when I visit a home, scanning spines, noting the titles & authors.
Yes, it can tell you a lot about a person. Condition of the books also matters. Have they been read (and re-read) or are they merely a purchased book collection to show his “intellect”.
I do the same thing. One can equate a bookshelf to a mental pantry....bread for the head, so to speak.
I fully expect my guests to scan my bookshelves, also...sadly, most dont.
I am currently looking for a higher grade of guest to visit my home.
Or, like me, it can mean person who is careful with his books and takes care of them. A girlfriend of mine way back when was sitting next to me reading a paperback, and when she cracked the spine, she said it looked like I'd gotten an electric shock. I was horrified.
As for the main theme of this story, I'll tell an anecdote about some friends whose high school-age son wasn't doing well in school. They asked me how to encourage him to read more and I told them to have a house full of books. What I really wanted to say was that they need to demonstrate that they value reading, instead of watching sports on TV, which was basically all they did with their son.