Roman Mars has spent 10 years using his radio show \u201c99% Invisible\u201d to reveal the everyday quirks and delights of cities. Now he\u2019s co-written a book called \u201cThe 99% Invisible City: A Field Guide to the Hidden World of Everyday Design.\u201d Mars talks with DnA about tales from LA, writing for print v. radio, and whether he secretly yearns to be a designer.