For a 91-year-old, Willie Nelson is a pretty busy guy. On top of his still very active recording and touring career, the singer/songwriter from Abbott finds time to advocate for policies that decriminalize marijuana. Most recently, he endorsed a Dallas proposition that prohibits arrests for carrying four ounces or less of the substance. (It passed.) He also owns a cannabis brand called Willie’s Reserve. It should come as no surprise, then, that when Willie and his wife, Annie, set out to write a cookbook, there was only one topic to cover.Out now, Willie & Annie Nelson’s Cannabis Cookbook (Simon & Schuster) is a combination food-travel memoir and primer on cooking with cannabis, and I learned a whole lot while reading it, from the couple’s favorite…