BY RAY McALLISTER It’s the Ides of March, which may be a fitting day to announce here that I’m leaving Boomer magazine after 11 years — eight as editor, three as columnist only. Nothing wrong with Boomer, obviously. (We won’t enumerate all that’s wrong with me.) But the book business that was supposed to be a sidelight has exploded. For instance, I: 1) just signed a contract yesterday with a writer to publish her book this fall, 2) will sign a contract with a novelist next month to publish his, 3) am redesigning another writer’s children’s book this week for reprint, and 4) have to update one of my own…