SAN FRANCISCO, Jan. 31 — A federal jury today found the author of marijuana books and advice columns, Ed Rosenthal, guilty of marijuana cultivation and conspiracy. Under mandatory sentencing laws, Mr. Rosenthal faces a minimum of five years in prison. Mr. Rosenthal, 58, who admitted to growing the plants for distribution under California's medicinal marijuana law, known as Proposition 215, called the verdict a "terrible decision" and vowed to fight it. His lawyers said they would prepare motions for a new trial immediately. "What the federal government is trying to do is destroy Prop 215 and eliminate medical marijuana from...