SALT LAKES: An Unnatural History. By Caroline Tracey. W.W. Norton & Company. 272 pages. $31.99. Earth harbors more than 100 salt lakes of varied dimensions. But they share one overriding feature in common: almost all of them are at risk of going dry, with significant ecological consequences. Science, memoir, culture and travel writing merge, not always seamlessly, in Tracey’s “Salt Lakes.” Journalist and geographer Caroline Tracey’s subtitle reveals her approach, and it’s rather a mixed bag.