The Intelligent Investor by Ben Graham ought to be required reading for every private investor. While the innovations he brought to stock analysis have long been outdated and the red flags he used to watch out for in a company's accounts are now regulated against by the SEC, many of his insights about thinking about investment still stand. For example, his description of Mr Market is still an excellent way of understanding how a crowd moves with the daily news.
If there are any lessons to be learned from the American sub-prime mortgage crisis, the 2008 stock market crash (information here) and Wall Street bailout that followed - and there are lots of lessons - it is that borrowed money can be very dangerous in investments, even when it is being handled professionally. The failure of LTCM, Bear Stearns, Lehman Brothers, Northern Rock and many others shows just how precarious a business model can be with too much gearing.

Dollar-cost average: This sounds complicated, but it’s not. Dollar-cost averaging means investing a set amount of money at regular intervals, such as once per week or month. That set amount buys more shares when the stock price goes down and fewer shares when it rises, but overall, it evens out the average price you pay. Some online brokerage firms let investors set up an automated investing schedule.