One excellent source is the letters that Warren Buffet has been writing each year to his shareholders. You can access these at: http://www.berkshirehathaway.com/
From his latest, I found this tidbit, which was sound advice:
Investing is an activity in which consumption today is foregone in an attempt to allow greater consumption at a later date. “Risk” is the possibility that this objective won’t be attained.
By that standard, purportedly “risk-free” long-term bonds in 2012 were a far riskier investment than a longterm investment in common stocks. At that time, even a 1% annual rate of inflation between 2012 and 2017 would have decreased the purchasing-power of the government bond that Protégé and I sold.
I want to quickly acknowledge that in any upcoming day, week or even year, stocks will be riskier – far riskier – than short-term U.S. bonds. As an investor’s investment horizon lengthens, however, a diversified portfolio of U.S. equities becomes progressively less risky than bonds, assuming that the stocks are purchased at a sensible multiple of earnings relative to then-prevailing interest rates.
It is a terrible mistake for investors with long-term horizons – among them, pension funds, college endowments and savings-minded individuals – to measure their investment “risk” by their portfolio’s ratio of bonds to stocks. Often, high-grade bonds in an investment portfolio increase its risk. (Emphasis mine)