Just as Robert Langdon gets ready to take his fans on another thrill ride in the forthcoming new Dan Brown novel, “The Lost Symbol,” another tantalizing mystery has popped up: Is Asus getting ready to unveil a low-cost, dual-color-screen e-book reader just in time for the holidays? A report quotes a UK Asus executive as saying the company, manufacturer of what could arguably be called the first netbook computer, would introduce a device before the end of the year that skirts the line between netbook and book reader.