I've read:

The Hitchiker's Guide to the Galaxy (Twice)

Twice? TWICE? I ended up destroying my first copy through sheer overreading, and my current copy is now a bit dog-eared too.

Everyone should read the radio series script books, BTW. Several bits got cut or shortened to fit the radio timeslot, including one of my favourite lines, Arthur in Milliways saying "So you mean the Universe doesn't end with a bang but with a Wimpy?"

Peter