I read Everything is Miscellaneous last year and thoroughly enjoyed it. It generally puts forth that hierarchies are outdated as indexing schemes. They are useful, and please understand that I'm taking a few liberties here, only as long as they are tied to a physical index or item.
As an example, It begins with a study of Office Max and their own study of space. A traditional store, most especially a grocery, is organized hierarchically and with the perishable staples at the back so that you are directed through a majority of the store most of the time. Instead, Office Max puts the most used items, toner and paper, at the front so as to make the average visit the most efficient. Their shelves are no more than 6-ish feet high, so that everyone can see the broad signs that they have identifying each product group. But the big thing is that they store some things in multiple places. Printer cables go next to the the printers, with the cables, and at the quick grabs section up front. That's it, that's where they break the hierarchy.
It's pretty minor, in fact Office Max limits each item’s number of locations to a small amount, but there are multiple locations available and that's a start. You see, a hierarchical structure requires a single location in the tree, but things don't fall that way. My dad's dog is in the genus Canis and the Kingdom Animalia, but it also is some dam's pup and as such has a family tree (another hierarchy this one multideminsional, we all have two parents, as do they, as do...), his name is Buddy (a member of a non-structured, unbounded set; see The Artist Formerly Known as Prince), he is black, brown, and white (all members of a relatively unstructued set), he is happy, rambunctious, and contributes to the happiness (and sometimes bloodiness) of the house. Well you get the idea.
So where do you put him in an Encyclopedia and represent all those things. You could put him in a traditional place and add cross references, but where do you end. Pretty soon the cross referencing becomes a good portion of, and then more than the entry itself. But depending on how you are looking for something about him, 'dog' can be a long way from where you want to get and that ignores the necessity of two-way references to make some of those traversals possible. So do we give up? No, each of those descriptors is adequate, so we use them all, structured and not, bounded and not. We create a multideminsional description of what we know. You'll regognise a simple version of this from tagging available in many e-mail, blogging, social, and wiki software available today. It's also the relationships that we describe in them. Our friends literally describe us. Tags are a large part of the 2 in the despicable term Web 2.0.
To prove these relations, and their power, I must only mention Mork. From that I know we recalled as much as we could. And most importantly, we’re happy.