AT THE GOING DOWN OF THE SUN by Elizabeth Darrell (St. Martin's: $14.95). "The village of Tarrant Royal had taken Rex Sheridan into its heart from the day of his first childhood escapade. . . . Roland Sheridan was known as a very fine horseman and rider to the hounds, yet it was into horses' ears that he spoke of his innermost longings and emotions. . . . Christopher Sheridan would have had no difficulty finding maids to roll in the hay. . . . With a gasp she put her mouth up eagerly to meet his as he bent his head and almost immediately those remembered mounds were pushing against his shirt. . . . 'You can forget Cambridge and a brilliant scholastic career . . . and I won't have any bastards bearing our name. You'll do the decent thing and give your child the heritage it deserves.' " That's all anyone needs to know about this book, one of the heftiest pieces of tsk-tsk, oh-my and romantic writing to come down the literary lover's lane since Elizabeth Darrell changed her name from Emma Drummond.