INTO THICK AIR: Biking to the Bellybutton of Six Continents (Sierra Club, paper, $16.95) starts with a novel conceit: Jim Malusa, a Tucson-based writer and adventurer, decides to travel to the lowest-altitude points on six continents to make an online documentary series for the Discovery Channel. Over the next six years, he pedals his bicycle through Patagonia, the Australian desert, the Dead Sea basin, the shores of the Caspian Sea, Death Valley and the Lac Assal region of Djibouti. Malusa’s human encounters are mostly pedestrian, and he spends too much time recounting preparations for each of his journeys, including several days of negotiation to retrieve his satellite phone, seized by customs officials in Cairo. But his descriptions of desert landscapes can be extraordinary. Lac Assal, in East Africa, is “a bull’s-eye of deepest blue ringed with purest white, sunk far below in a black spill of lavas from the volcano.” On the empty track to Lake Eyre in the Australian outback, deposits of stone and gravel “glow like burnished copper.” Poking through them “are skeletons of saltbush, quivering in the wind. I push on, chattering across the rocks, plowing through the sand, crossing a basin of dried mud aglitter with half-buried gypsum crystals.” You can almost feel the dry gusts turning Malusa’s lips into cracked leather.