Atmosphere

Be sure you have enough chairs and pillows on hand to keep your guests comfortable -- just because you're reading doesn't mean it has to feel like school. Decorating for your book club depends a lot on what type of book you read.
For a gothic novel: keep the lamps on low and light a lot of candles instead.
For a tear-jerker: decorate with tissue boxes and aromatherapy candles.
For a crime thriller or mystery: recreate the game Clue-- place props such as a rope, knives, candlesticks, wrenches, magnifying glasses and other whodunit items weapons peaking out from drawers and bookshelves -- anywhere they'll be noticeable but not actually injure somebody.
Visit your local library and decorate the shelves and coffee table with books from the author you're reading or biographies about that author. Not only will the books help set the mood, you can thumb through them to look for topics to help your discussion.