Call it a byproduct of the American car culture, a side effect of the rush to multitask, or simple consumer laziness: whatever the cause, sales of audio books are on the rise, and the publishing industry is taking notice. One installment in the Harry Potter series sold 575,000 audio copies in three days, and Hillary Clinton’s recent autobiography has sold 90,000. Audio sales are still miles away from print sales, of course, but the industry is beginning to treat audiobooks as a serious part of its product line.