Two years? Three years? Five years? It's a parlor game in publishing circles to speculate how long it will take before e-books constitute a majority of their industry's sales in the U.S. But whenever that tipping point is reached, no one doubts that's where the market for readers is headed. Amazon, the industry leader, already sells three times as many Kindle e-books as hardcovers. Other book sellers aren't far behind. Last month, the Association of American Publishers announced that for the first time monthly e-book sales had overtaken hardcovers in January. Paperbacks remain in the lead, for now. For the complete article at CNN.com, click here.