Anita Leslie's Mrs. Fitzherbert (Scribner) provides a frame of reference to many readers only hazily familiar with this period in England's social history, and current interest is consequently more lively than it would have been of recent years. For these letters from Richard Sheridan's sister, written to another sister in Dublin (1784-1786-1788-1790) are the very stuff of which that social background was made. It has a kind of period charm, a naivete, and it gives the reader a sense of being in on what happens in the households of those related to a public figure. Betsy was her semi-invalid father's constant companion, and the scene shifts from a London with many social and dramatic affairs -- to the watering places where they went for his health. It is fragmentary, often repetitious, but the kind of off-beat thing that might secure a carriage trade in the knowledgeable circles.