First of all, to explain the title-it is an engineering term that describes the angle where dirt/soil settles after being piled up. More about that later.
The novel is narrated by Lyman Ward, the grandson of Oliver and Susan Burling Ward. Lyman was a retired professor of history who was divorced and confined to a wheelchair. He decided that he wanted to write a biography of his grandmother and he went to live in his grandparent's old home in Green Vally, California, much to the displeasure of his son, who felt that Lyman would not be able to care for himself there. But the caretakers of the house helped Lyman daily and he was determined to stay there. He had old letters/correspondence between his grandmother and her best friend, along with old articles from magazines and newspapers and so he began trying to put together the history of the family from when they went out West a century earlier.
As Lyman went through the old letters, he began to learn more about Susan Burling Ward, his grandmother who had raised him. In 1868, Susan was an educated, cultured artist, a young Quaker woman living in the East when, on New Year's Eve, she met Oliver Ward, a young ambitious sounding man who wanted to head West and become an engineer. A week later, Oliver left for the West and Susan did not see him again for five years. The did write to each other and over time an "understanding" came to be, so that when Oliver returned to the East they agreed to marry.
And thus the lifelong journey began. The family moved around in the West through-out the years as Oliver worked on mines and canals. Susan helped to support the family with her art and writings. They had three children.
This is just such a simplified summary...there is so much more in the book. Some of the most influential characters were Augusta Drake and her husband Thomas Hudson. Augusta was Susan's best friend and it was the letters between Susan and Augusta that Lyman was using to write Susan's biography. And Frank, Lyman's assistant, who was in love with Susan. It was never made clear exactly what Susan and Frank's relationship was, but clearly it was a love relationship, if not a sexual relationship.
As Lyman worked through all the letters and articles and memories of his grandparents, he began to study the story of his life and his marriage. Near the very end of the story, Lyman had a very disturbing dream that included his ex-wife:
"'What do you mean Angle of Repose?' she asked me when I dreamed were were talking about Grandmother's life, and I said it was the angle at which a man or woman finally lies down. I suppose it is; and yet it was not that that I hoped to find when I began to pry around in Grandmother's life. I thought when I began, and still think, that there was another angle in all those years when she was growing old and older, and very old, and Grandfather was matching her year for year, a separate line that did not intersect with hers. They were vertical people, they lived by pride, and it is only by the ocular illusion of perspective that they can be said to have met. But he had not been dead two months when she lay down and died too, and that may indicate that at that absolute vanishing point they did intersect. They had intersected for years, for more that he especially would ever admit."This novel covers over 100 years and four generations of the family. Fascinating book!