I've mentioned before that I've long held a fascination with taxonomies, classifications systems and the like.
I've also been a long-time advocate of hierarchy being derivative (via ordering of facets) rather than primitive, and of faceted tagging (back in 2005 I called it tag the tags but see Google Code Project Hosting for a great example of how simple tag structure achieves this).
So I'm having a great time (half way through) with David Weinberger's Everything is Miscellaneous which is a wonderful book about these kinds of topics, not just in the computer age but going back to topics like the alphabetization, library catalogs in Alexandria, the Dewey Decimal System, the Periodic Table (and alternatives) and the Linnaean Taxonomy.
The book doesn't fall into the books that changed my mind category but more the I never thought of putting it that way before category.