One might be tempted to dismiss salt as an odd subject but it has been an important driving force in trade for much of history. Enough so that the author, Mark Kurlansky, wrote a nice thick book of about 500 pages. It's interesting, assuming you like a gastronomic view of human history. It covers the ancient brine wells of Sichuan, where the Chinese harnessed natural gas furnaces about 1500 years before westerners. It talks about the links between the availability of natural salts and cheese regions of Europe. And there's the importance of salt for food preservation and how refridgeration led to the devaluation of salt as a commodity.
It's a fascinating book but also a thick tome through which one progresses slowly, with many incidental anecdotes and curious details. I somehow picture it as a Victorian book: thick, of an odd subject, and filled with stories as much as fact. The sort of book you'd picture Dr. Watson perusing on a wintry evening.
Anyhow, it's quite good, if you like that sort of thing. Here's a selection (somewhat edited) from the conclusion:
"After thousands of years of struggle to make salt white and of even grain, affluent people will now pay more for salts that are odd shapes and colors. Many people do not like Morton's idea of making all salt the same. Uniformity was a remarkable innovation in its day, but it was so successful that today consumers seem to be excited by any salt that is different. Gray salts, black salts, salts with any visible impurities are sought out and marketed for their colors, even though the tint usually indicates the presence of dirt."