Raph Koster's A Theory of Fun has been out of print and nigh-unattainable for quite some time now. I've had an outstanding order for it with Amazon.com for something like six months. This seems to me to be one of the few indispensable books on game design out there, so this situation has been rather depressing.
But today brings great news! Raph Koster himself writes that the book is going back into print, and should become generally available again sometime soon:
I now have official word that A Theory of Fun for Game Design will be published by O’Reilly. They are going back to press on it pretty much immediately to meet the giant pile of backorders (like, well over a thousand), so hopefully everyone waiting for a copy will be able to get one for less than $200 in the next few months.