There are 253 passengers on a seven car Tube train that is about to crash. Every person, along with their thoughts and actions on their brief train ride (and including footnotes explaining their direct and/or indirect relationships with other people on the train), is described in exactly 253 words each.
While on the surface this may sound like nothing more than a mildly interesting experiment in constrained writing, the book manages to reach a deeper meaning than you would expect. Whether you read the book from beginning to or flip around to random parts at your leisure, the overall effect is the same; allowing you to freeze a moment in time and examine the lives and deaths of 253 people with more in common than they will ever truly realize. Contrasting and comparing their personalities and motivations affords the reader an almost God-like chance to examine the fantastic and mundane worlds of a train full of strangers as an intrinsic whole.
But don't let that scare you away. If you rather enjoy as a distraction rather than a perceptions-enhancing experience, it easily works on that level as well. No matter how you attack 253, it remains a truly unique book in both structure and subject matter, and equally enjoyable whether read in short bursts or cover to cover.