This week on “Down the Stacks,” I’ll be looking at a six-volume military epic written on a bet by Jim Butcher, of Dresden Files fame. After a debate about whether or not a good author could produce a good book from a bad idea, Butcher bet he could write a good book based on two random ideas. The concepts given to him were a lost Roman legion and Pokemon, and the books that came forth are collectively called The Codex Alera.