Though not primarily concerned with Game Design,
Well Played is an interesting look at the value and meaning behind games (as well as some cool looks at design philosophy). Though sometimes a bit heavy-handed on academics (the price you pay when academics are involved... it's their culture), essays offer a different look at games. Topics include everything from dramatic meaning in
Ico and
Shadow of the Colossus, why
Bioshock's story is a failure and a breakdown of the design behind
Super Mario Brothers. I can't say I agreed with everything, but even the ones I didn't agree with made me rethink my approach.
While not every day forum-fodder, the essays are worth reading. Besides, if you're the type to constantly fight under the banner of "Games as art," its good to have something intelligent and meaningful to bring to battle instead of the tired, overly used battle cries we've already heard ("Games have music and art, therefore they are art."; "I cried like a baby when Aeris died, I was moved and games are art."... you know them, so chime in...)
I've read several books similar to Well Played, but this is one of the few that manage to discuss games in an intelligent way while also managing to keep their original intent, the fun, front and center.
You can download Well Played for free at the link below. If you'd rather something non-digital, you can purchase a copy from Amazon.