As I (along with everyone else, apparently) loved Air, I was intrigued enough to pick this up in a charity shop. It's an 'internet novel'. What's that? It's something first published online that has since been rejigged for paper printing. Is much added? Dunno - I haven't checked the original, but I can't imagine there's much to the transformation.
What is it? 253 short stories, in the form of character sketches, including what they look like, and what they're thinking. Each is 253 words long. The 253 people are the passengers on a tube train a couple of stops before it crashes (I guess some drama is needed!).
What does the book think it is? I guess this is best indicated by the joke 'questionnaire' at the back of the book (there are plenty of these fake advert-style interludes throughout the book - none exactly sidesplitting): 7.) As I understand it the significance of 253 is (one answer only, please):  as a metaphor for impending death as we live life  as a tribute to the hidden fascinating truth behind each face if we could but find it  as a tribute to the infinite variety of London life  as a deliberate mockery of interactivity, feedback forms etc.  that some of it is mildly amusing  that Geoff had a contract.
I guess 'all of the above' would be pretty fair. It's a pretty bitter-sweet book. There's some good comedy in a few of the entries, but often it's at the expense of the characters. Likewise, there are some very touching entries, but they're mostly not uplifting either. In all, rather depressing for a vaguely comic book. More fundamentally, does it work? Barely.