Lord of the Rings -- Book or Trilogy?

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I'm in the middle of a heated argument with a coworker battling it out whether The Lord of the Rings is considered a single book (as he contends), or a trilogy made up of three individual books (as I contend).

I'm an editor for Borders.com and I'm trying to decide how to handle Lord of the Rings in text: italicized or not.

Any body have any answers?

Many thanks!

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