ONE book?!? Ha! Hitchhiker's Guide to the Galaxy Series, Douglas Adams, Dirk Gently's Holistic Detective Agency, Douglas Adams, Myth Adventures (well, pretty much the whole series), Robert Asprin, Citizen of the Galaxy, Robert Heinlein, Glory Road, Robert Heinlein, Any of the Discworld Series (except Small Gods), Terry Pratchett.
2. What is your favorite genre?
Humorous Science Fiction/Fantasy (as indicated by the above list).
3. Do you usually buy your books or visit the library?
Library (although I own much of the above list).
4. Who is your favourite author?
Terry Pratchett (but I come in a close second; does that seem conceited?).
5. What book have you read that you absolutely hated?
Lord Foul's Bane, by Stephen R. Donaldson.