Charlotte Stretch, whose column in The Guardian gave me this idea, tried it with Five books she'd already read to see how it worked out. I have done the same with mixed results.
A Study in Scarlet by Sir Arthur Conan Doyle
Madame Charpentier is relating the details of a case, complete with a mysterious death, to Sherlock Holmes, who comments sarcastically "It's quite exciting...What happened next?"
Yes, that gives you a pretty good feel for the book.
Going Postal by Terry Pratchett
Moist Von Lipwig is conversing with the Golem, Mr Pump. Not the most exciting portion of the novel, nor the funniest, but it does leave you wondering how Von Lipwig came to be Postmaster and why he apparently has a golem.
Orlando by Virginia Woolf.
Page 69 has a lot of very poetic description and rambling about the nature of love, which doesn't tell you much at all about what the book is about but gives you a very good idea what kind of writing stlye to expect.
Harry Potter and the Deathly Hallows by J. K. Rowling
Spoiler warning!
Harry, Lupin and the Weasleys learn of the death of Mad Eye Moody.
So I suppose that gives you an idea that there's going to be death and sorrow in this volume.
The Adoration of Jenna Fox by Mary E. Pearson
One of the characters is explaining how she lost all her limbs to an antibiotic resistant bacterial infection, and that what seem to be her real arms are actually amazingly good prosthetics, complete with her original freckles. This gives you an idea of the medical and bioethics themes in the book but doesn't tell you anything about the plot or the protagonist. I think it would be enough to tell me that I would like the book.