THAT man of letters, George Adams, calls us from his North Point den with a reminder that The Great Hong Kong Sex Novel, his latest contribution to the cause of English literature, will be out next month. Glancing through Keeping Posted's review copy we notice that novelist Adams doesn't waste much time in, ahem . . . getting down to it. The first two chapters bear the racy titles Night in a Girls' School and Candy Yeung and the Empire of Legs. Says Adams: ''There are another 14 chapters like that so hold your breath.'' (Though he should try telling that to the heavy breathing brigade who are great fans of his steamy tomes.) Adams goes on: ''I'd be interested to hear what people imagine the chapters are about before they read the book. A free copy of the book to anyone who can guess correctly.'' Keeping Posted's copy, meanwhile, is eagerly doing the rounds of the office.