Yoko Ono held a brief — but very secure — book signing for “Acorn,” a follow-up to her 1964 work, “Grapefruit.” She arrived at the Refinery Hotel with four bodyguards and gave an introduction explaining she wrote the book “in short bursts since people no longer have full attention spans.” “One woman shouted ‘Feminist icon!’ while she was speaking,” says a spy, who adds Yoko was whisked out after 20 minutes. “Acorn,” a book of “instructional poems” and drawings will be published on July 15.