I want to jot down a few thoughts about the book while they're fresh (ish) in my mind.
I think that description of if as 'A weirdly hypnotic tragicomedy of the banal’ posted upthread is perfect. It is filled with quotidian details, but I don't find these banal at all. I love the bits about clothes, or meals. And I did find it strangely hypnotic.
I think there are lots of comic touches, and I found Mrs Cutler very amusingly drawn ('Never say die!'). I think there were also some elements of the ridiculous - aspects of the relationship between George and Mrs Jacobs, for example - that I found funny.
I agree that the theme of disappointment and unfulfilled potential runs through the novel, but like many of Brookner's protagonists, Ruth is also quite steely and self-sufficient in many ways. And though her start in life is far from ideal, she has at least achieved moderate academic success by the end.
One thing I've noticed in Brookner's novels is how she often shows an isolated woman who is sort of brought to life by proximity to a happy and more glamorous couple - in this novel, Hugh and Jill. I recently read Look at Me and a similar thing happens there.
As for Helen and George, they are truly dreadful characters - self-centred, vapid, and as a pp said, so vain. I was frustrated by Ruth's sense of duty to them - though perhaps that's why it could be read as dated? I don't think many young women today would be quite so willing to give up their lives and careers in this way - thank goodness.
I also found the subtle references to Mrs Weiss's Jewish background very interesting, and assumed the implication was that she had fled Nazi domination in perhaps the 1930s.
Lastly, I've never read any Balzac but feel inspired to, and wonder if I had, the novel would have meant even more.