answer:It’s a symptom of suspicion. And that suspicion, rightly or wrongly, comes from your observation of your mom and the other husband. The book is dated 6 years before your knowledge of your parents being friendly to the other couple. That’s pretty sketchy evidence, and I would believe your mom not remembering the provenance of the book dated 19 years ago. If your mom and he were having an affair, and it was that old, it’s unlikely that it would have evolved into a couples friendship between the two couples. How long have your parents been married? How long has the other couple been married? None of this I have written is a statement on your suspicions based on current events and activity, just that there is no evidence from the cook book. But unless you see something blatant to move you to confront your mother, I would keep your suspicions to yourself.