AND THE BEAT GOES ON

AND THE BEAT GOES ON Sonny Bono (Pocket Books, $19.95) The title is apt. You'd have thought by now that on talk shows and in gossip sheets the self-described ''short, mustachioed, Italian half of Sonny and Cher'' would have milked his marriage for every last scurrilous detail. But, no, Bono is still bitter enough to deliver the lowest possible blow-an attack on Cher's deficiencies as a mother. That charge, plus the usual whining about how she ''walked off with the franchise'' after all his hard work, pretty much sums up this air sandwich. F