NO SIGNPOSTS IN THE SEA by Vita Sackville-West (Penguin/Virago: $6.95, paperback). Written as a diary from the pen of a dying man, Vita Sackville-West's last novel presents the dynamics of a perfect love story. Reissued as a "modern classic," it is worth reading for two reasons: A period piece, it describes the remnants of pre-World-War-II English society, now obsolete, whose snobbish views of the working class explain a good deal about the state of Britain today; on another level, it is a eulogy to companionate marriage. The bachelor diarist is secretly smitten with a beautiful 40-year-old widow. On discovering that she had booked herself onto a cruise, he signs on too, determined to spend his dying days with her. If we did not know as much as we do about Sackville-West's own remarkable marriage to Harold Nicolson, the conversations between the diarist and his inamorata might not be interesting. But we do know that both Sackville-West and her husband found sexual fulfillment in homosexual relationships, each following his or her independent ways, while at the same time maintaining their marriage. Although Sackville-West is undoubtedly idealizing the peculiarities of her own marriage, her perceptions of the constraints that certain traditional marriages place on all women ring true. The unrequited love story she tells is well set at sea. There the experiences of the moment are beyond space and time. The experience is precious in itself. It doesn't matter that at sea there are neither milestones, nor tombstones.