The ebullient ``Dr. Ruth'' broadens her definition of marriage to include those who ``live together well, even if they are not legally tied'' but are willing to commit themselves to one another sexually. She covers a lot of ground here, in less detail than most reliable sex manuals, but the difference is in the delivery. The book is transcribed from dictation in the voice of the self-proclaimed ``Grandma Chickensoup,'' who has become familiar to millions through radio and television. She dispels popular mythology (``there are still lots of virgin honeymooners''), and she is funny (``if just completing the sex act is a challenge, don't try it on a waterbed''), techniques which are therapeutic in themselves. This book will appeal to her already wide audience and may attract others as well. Suzanne Druehl, Little Rock P.L., Ark.