This is an extremely abnormal and inappropriate situation. Such a scenario involves unethical and immoral behavior that goes against basic family values and social norms.
Just smile and share a little summary of the book with her. Maybe she'll join you in reading it later. For example, if it's a book about adventures, you can tell her some exciting parts.
Immediately show it to her with pride. Point out the parts you like the most, like 'Grandma, look at this color here, I think it makes the whole picture pop.'
I'm not sure specifically what this 'grandma sees cock story' is. It could be a very unique family - related or countryside - type of anecdote. Maybe it's about a grandma seeing a rooster in a yard and something interesting that follows from that encounter.
Forgiving him depends on a lot of factors. If he is truly remorseful and willing to work on the relationship, it might be possible. But it won't be easy and will take time.
Once upon a time, Grandma had a big farm. There was a giant cock on that farm. It was the most beautiful and biggest rooster around. Every morning, it would wake Grandma up with its loud crowing, and Grandma would go out and feed it. The giant cock was very proud and would strut around the yard all day.