WEDNESDAY Fast Company’s Charles Fishman has a dirty little secret: he loves him some savings at Wal-Mart. It makes him all hot and bothered. But he feels really guilty about it, because, apparently, the retail behemoth may have certain deleterious effects on the marketplace. Who knew? Join Fishman as he discusses The Wal-Mart Effect at Politics and Prose. 5015 Connecticut Ave. NW, 7 p.m. THURSDAY Pint-sized author and newly minted voiceover star Sarah Vowell will…