I'm a big Gordon Lightfoot fan myself. Amazon had a deal on all but his first four albums for 7.99 as mp3s, so i bought it and began enjoying his work tremendously. I would start one of three ways:
His first four albums are available as a double CD for 10 bucks. While I think his middle years stuff is the best, these first four are a great way to get started, and his first album is one of my favorites.
His greatest hits is a great 20 song set covering much of his career and lots of great, well-known songs. Can't go wrong with a smattering of a bit of everything, right?
If you want a specific album to start on, I think Sundown is his most famous, and has a bunch of great songs. I also think Sit Down Young Stranger would be good to start on (renamed to If You Could Read My Mind in the US)
Good luck plumbing Gord's gold :)