OK, so a few things were fundamentally different in the book that affected the ending.
Towards the end of the book, Margaret agrees consider going back to California over the summer, but only for a few weeks holiday. The family are all planning to continue living in Hong Kong, so that was completely different. The fact that she is willing to contemplate leaving Asia for a few weeks shows that she is starting to move on and accept that they will probably never know what happened to their son and that she needs to keep living. Note, the trip to the US is a future plan only - it does’t actually occur in the timeline of the book.
In the book Hilary was Caucasian and she was desperate for a baby and had been trying for years (the bit about her going back on the pill completely changed her character’s struggle and motivations). She had been spending time with a (mixed-race, Chinese-Indian) boy from a children’s home with a view to adopting him. She was doing this without David’s involvement and at the end even though they weren’t together, she asked him to participate in the official adoption because she thought she probably would be not allowed to adopt as a single/divorced woman. David agrees to help her adopt. She’s planning to move back to California once the adoption comes through.
The final scene to the book is similar to the series. Margaret and Hilary visit Mercy. Mercy has had her baby and is planning to stay in HK (no, the visa issue is never mentioned!). They are supportive and kind. She tells them David has not been to visit. Margaret has forgiven Mercy for her role in G’s disappearance. Hilary seems to hold no grudge against her either for having an affair with David (their marriage was pretty much over anyway). The baby seems to represent hope for the future and the final meeting is an act of female solidarity and support.
Note, Hilary is independently wealthy (family money) and doesn’t work in the book. Given her wealth, she could probably stay in HK with some kind of visa not attached to a job. Another detail is she and Margaret knew each other vaguely from when they were in school, but weren’t close then. Hilary also had been a chubby kid and her obsession with staying slim is a big theme for her character.
Another side note, in the book the child is only ever referred to as G. We don’t know his full name. The name Gus is only in the TV show.
Throughout, Margaret has sessions with a therapist, and I think this really could have added a lot to the series. The conversations about her coping (or not) and learning to move on would have been a way to express a lot of what was going on inside her mind.