Proposals on this hot topic are blanketing editors' desks, but in a few days, HarperPaperbacks' instant book Heaven's Gate: A Cult Suicide in San Diego will be the first to hit stores, thanks to speedy action by HarperPaperbacks editorial director Carolyn Marino. Marino signed up writers at sister company the New York Post as soon as the news of the mass suicide hit, and the manuscript was sent off for its 500,000 first printing last Friday. The book is the first to be distributed via the house's new arrangement with Murdoch Magazine Distribution.