It sounds like Alberto Manguel has a hell of a library. He rebuilt an old barn just to house his 30,000 books. It’s where he begins his book The Library at Night. The title refers to the change he experiences at the end of his reading day: “But at night, when the library lamps are lit, the outside world disappears and nothing but this space of books remains in existence.”

The Library at Night is a book of essays about libraries. Manguel constructed most of them from a generous handful of anecdotes clustered around one or two topics on which he goes deeper. Every essay looks at one of the functions of libraries.

What do libraries do? Judging from libraries’ websites the most popular answer is the kind of banally lofty statement of purpose normally written by committee (“The UC Berkeley Library connects students and scholars to the world of information and ideas”). This is also the boring answer. Manguel isn’t writing about the day-to-day business of libraries but about their purpose.

The essays fall roughly into three categories. Sometimes Manguel writes about how libraries order information: not only how the books are ordered, but how the space works. Sometimes a library’s architecture leans over your shoulder and tells you how to feel about its books. An 18th-century concept for a library drawn by Étienne-Louis Boullée looked like a cavernous train station. A place like that would pressure you to flip through a book, scribble a couple of notes, and move on.

The other categories are about libraries as cultural institutions and libraries as expressions of individual minds. Mostly, Manguel is examining libraries as physical embodiments of ideas—public libraries reflect the aspirations and ideals of a group, a private library is an expression of a single person’s mind. It’s common to jokingly refer to modern technology as an “outboard brain.” In reality, we’ve had outboard brains as long as we’ve had books.

Manguel ends the book with “The Library as Home.” That third category of essay, about the library as a home for the mind, got me thinking about my own library. It’s a haphazard, unsystematic collection, actually. There are books I like, or want to read, that I’ve never bothered to acquire. On the other hand, I’ve bought a lot of books because they were sitting on remainder tables, looking interesting and flaunting seductively cheap price labels. (Let’s not mention used bookstores. I’ve had to stop going. I can find anything online these days, and it’s guaranteed to be what I actually wanted, and not an impulse buy…) Often these purchases turn out surprisingly well; sometimes they’re just books I might as well have borrowed from the library. I wonder, having read Manguel’s book, just what my half-random collection says about my mind.