It’s unlikely to be Amazon, which is forging its own path and has thus far seemed only marginally interested in physical book retail. It’s not clear that Books-a-Million and Half-Price Books, the second- and third-largest general trade book retailers, have the resources, let alone the ambition. One possible candidate, according to multiple sources in book publishing and retail, is the Canadian book seller Indigo, which has defied the bleak trends in publishing in recent years, posting profits and selling literary fiction by the crateload.