In a nutshell: categorization is hard. Libraries have struggled with it for centuries, and we continue to struggle with it today. Whenever you are forced to choose a single label for an object or concept, it becomes hard to find by way of all the other labels you could have used. With computers and databases, things are considerably easier, because you can tag things in as many ways as you can imagine and then retrieve them with a simple search. Sometimes, you can even get masses of people to tag things for you, which can save you a lot of effort.

I read this book several months before writing these notes, so I'm almost certainly not being fair to it here, but I remember finishing this book thinking that there wasn't much of interest beyond what I'd already learned from using flickr and reddit.