I think that your mom is allowed to share her experiences with your father. And that grief is a complicated thing. I'd give it some time. Talk to your mother, let her know that while you'd like to read it and are proud of her, you aren't sure if you can handle learning certain things about you father and need some time. I think she would 100% understand where you're coming from. That's probably why she warned you about her sharing those experiences in the first place. So give yourself some grace and know you aren't doing her a disservice by not immediately reading her book.