I'm a big fan of David Mackay's Sustainable Energy---Without the Hot Air, not least because it's a command performance by a master of back-of-the-envelope calculations. This post by Kent Beck on NoSQL databases is a smaller example of that art, but still fun. Amid quotes like, "[Amazon] EC2 is basically a really complicated way of charging for electricity," and, "At internet scale, programmers are (sometimes) cheap compared to the cost of electricity," there's some nice number crunching to explain why non-relational databases are suddenly fashionable. Thinking like this really does deserve to be called software "engineering".