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 the industry’s sales in the U.S. But the tipping point aside, no one doubts that that is where the market 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 week, the Association of American Publishers announced that in January, for the first time, monthly e-book sales had overtaken hardcovers. Paperbacks remain in the lead, for now.