I highly recommend The Official Tour de France: Centennial 1903-2003. It's the best book in English about the Tour that I've seen (I hope to get Peter Clifford's 1965 book someday). The photographs alone would make it worth the $30 price. But since the book was produced in collaboration with L'Equipe, they made excellent use of the archives of that paper and have great contemporary articles on each Tour.
Here's a review:
http://www.cyclingnews.com/
Colorado Cyclist has it on sale this week for only $22 ($8 off the cover
price): http://www.coloradocyclist.com/
Mordecai Silver
NY, NY