Book about focusing on what you have instead of what you lack

A friend of mine has a nice house, the family she always wanted, a well paying job ... satisfied or happy she is not. Always focuses on what she doesn't have or what's going wrong. Negative feedback in her job means she's "useless as a human being", her husband forgetting something means he's neglectful etc etc.

I'm looking for books I can recommend to her to switch her mindset to stop giving so much weight to the negatives.

Books that will help me make sense of it would help too, as I'm super at a loss on how to talk to her when she comes to me with one of her "negatives".

Can be self help or fiction.

Thanks!