NEW YORK ( TheStreet) - Like bull and bear markets on Wall Street, investing guides come and go. Burton Malkiel's classic tome A Random Walk Down Wall Street, however, has stood the test of time. In fact, the book remains so popular that a 10th edition was recently released, adding to the more than 1.5 million copies already sold.TheStreet spoke with Malkiel about why his investing classic remains relevant. Why is "A Random Walk Down Wall Street" as important now as when it was first published? Malkiel: The thesis of the book was that people are going to be better off having at least the core of their portfolio in a low-cost, broad-based index fund, and that would beat most of the actively managed funds you could buy. I said this in 1973 before index funds even existed. Every time I do a new edition, I ask if it still works. And every time I find that two-thirds of active managers are beaten by the simple index fund. And the one third that win in one period are not the same that win in the next period. So I think the thesis absolutely still holds and it's the best advice I can give to investors. In the last few years, we have seen a government bailout, insider trading and high frequency trading. Is the market still efficient? It's efficient in the sense that it is still very hard, almost impossible to beat. Since it is so hard to beat, the one thing you can be sure about is that index funds have very low costs. And you can be certain that the lower the fee you pay to the purveyor of the investment service, the more there is going to be for you. We've seen a $50 billion valuation put on Facebook and the price of gold skyrocket. Are we seeing so-called "Castles in the Air" being created? We might be. But the problem is that you never know in advance which ones they are and how far they are going to go. Is gold a bubble? Are bonds in a bubble? Is China in a bubble? I don't think we know that either as individuals or professionals. And I think the best thing we can still do is hold a very broad based, full portfolio. A lot of people said that after the "flash crash" retail investors just gave up. Do you think retail investors are going to start giving up on the equity market? That is one of the problems. Retail investors generally buy when everyone is optimistic. They bought heavily just at the height of the Internet bubble in early 2000. They then sold at the bottom of the market in 2002. They sold in the third quarter of 2008. And they have been shocked, and they have not come back when they should have because the market has been very, very good. So unfortunately, and this is the problem with people trying to time the market, when you do that you almost invariably get it wrong. And this is not only individual investors but professional investors.