1) How does investing in metals protect against inflation?
The specific metals in question are gold and silver. Gold, in particular, has been used as money since money was created after the inefficiency of the barter system was recognized. The price of gold has nothing to do with the stock market. The price of gold in US dollars is a reflection of how much a single dollar is worth. In the 1930's, the price of an ounce of gold was $20.67, today it hovers around $950. That is a reflection of how much wealth we have lost, because our currency is not backed with anything. The chief difference between gold and dollars is that gold is finite. There is a certain amount of gold in supply in the world. If you are familiar with what inflation is and why it happens, it should make sense that this finite supply is what prevents the inflation of gold. Gold cannot be replicated like dollars. The fact that the dollar (a paper IOU) is not backed by a hard asset means that it represents debt that cannot be repaid. During the gold standard days, the dollar was redeemable in gold, and therefore, the debt that the dollar represented would be satisfied once redemption of the gold backing it took place.
2) When is it appropriate to invest in metals as opposed to other income-producing assets?
When some of us speak about investing in gold or silver, we are not making an argument similar to stocks vs. real estate, for example. Based on #1, gold and silver are wealth preservation investments. Investing directly in gold/silver does not produce income because the commodities themselves are stores of wealth. They don't build your wealth, they preserve it. They keep you from losing it. Now, gold/silver stocks, etf's, and funds are the means of building wealth that is backed by gold/silver commodities. In answer to your question, it is ultimately ALWAYS appropriate to invest in these metals. Other metals are debatable because it depends on their industrial uses.
3) How can I learn more about buying and selling metals?
Read a book that I am reading right now called Gold: The Once and Future Money. This book is about the history of money. Along with visiting websites that deal with buying/selling such as GoldSilver.com (Mike Maloney's (a rich dad advisor) site). Google the names Peter Schiff and Marc Faber. This is a start in the right direction.
More answers to your questions as you need them.