Ejnar Mikkelsen was like the Ernest Shackleton of Denmark. He was celebrated throughout the Scandinavian nation for exploring and mapping Greenland. It was a matter of Danish national interest, to preserve their hegemony over Greenland, to debunk potential American claims. Of course, explorers often died while on expeditions, as was the case with Shackleton (sort of). Mikkelsen and his Icelandic companion have good reason to worry that they too could come to a cold, premature end in Peter Flinth’s “Against the Ice.” To keep Greenland undivided, Denmark must prove it is one continuous land mass. Logically, that survey task has fallen to Mikkelsen. He was making progress in 1909 until his expedition partner’s mishap and subsequent frostbite forced them to return to the ship. An explorer like Mikkelsen cannot wait to get back on the ice, but he needs a volunteer from the shipbound crew to accompany him. Only Iver …