ONLY YESTERDAY by Julian Gloag (Holt: $15.95; 170 pp.). The action of this brief novel, the author's eighth, takes place over one weekend when three generations of the Darley family converge on the grandparents' rambling home in the English countryside. Oliver, the grandfather, is a grumpy, opinionated old man, an architect who still cherishes the dream of building a vast "pyramid," a climate-controlled utopia capable of supporting a population of 50,000. His wife, May, a gentle, intelligent woman, is physically frail, but emotionally strong. She has been advised to check into a hospital but is worried about leaving her husband on his own. The Darleys are expecting a visit from their granddaughter, Miranda, a medical student. They are surprised when their son Rupert (Miranda's father) shows up as well. He is worried about how they will take his latest news: He is leaving his second wife.