A London tube train, with every seat full plus the driver, makes 253 people. Describe each person in 253 words (outward appearance, backstory, what they're currently thinking), and you've got this book. (Well, actually you've got the 253 web site; the book I read is the print version.)
It's the relationships between the characters described that makes this book interesting. For instance, while reading, you'll notice that four of the characters have befriended different Big Issue salesmen — all is explained by the story of a character in the last carriage. There's also a recurring storyline which flips between several interpretations as you read different characters' viewpoints on it. It's written so that the stories unfold as you read from the front to the back of the train; if you're reading it online, resist the temptation to chase links at random.
The book also includes a load of spoof advertisements that fit around the book ("Learn the Secrets of the ancient Canadians", for instance, which teaches the reader how to generate random binary numbers using a coin — appropriate for a book with 256 sections).
Essentially, this book is "Life A User's Manual", but on a budget (and wandering around a Tube train rather than a French apartment building). It's not as beautifully-constructed as Perec's work, but it's a lot of fun to read and captures pretty much the same spirit.
The author's written other works with similar constraints. I'm not going to take much persuading to read more.