Do you think this is a sign of paranoia or just innate curiosity?

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.

Random input:

I buy books at used book stores. They often have notes on the inside, completely unrelated to me, that were written by whoever gave the book to its original owner.

A lot of ‘kids’ don’t know about their parents activities, and for good reason. What would happen if she had been having a relationship outside of the marriage and your dad was okay with it? See, lots of variables here. I’d just observe and try to trust my mom.

We’ve got a large population of swingers in our area (all unattractive if you ask me) and the kids don’t know at all. It’s called a party at the local hotel..ha!

That being said, my siblings didn’t even know their father had another child (me) until I contacted them when I was 18 years old, so some secrets are tougher than others.

I have a gold watch inscribed “All my love, Maria” I found it on the beach with my metal detector when I was 9 years old.

1994 was a long time ago now. For all we know this book was signed “From your secret pal” by the author of the book at a book signing, for some fan, who then later died and had all their stuff sent to a good will shop, where your mother then later purchased it.

You are not being paranoid, but you are not being reasonable and logical in your thinking.

This does not count as evidence.

I don’t think you have any evidence that your mother is having an affair. I wouldn’t call it paranoia, but it is irrational thinking.

If it is making you really uncomfortable and you are dwelling on it, it might be worth asking your Mom about it in a non-confrontational (and private) way. But you know your relationship with her best and what ructions it might or might not cause.

It is not enough evidence to develop a hypothesis. I would forget about it.

I would be curious if your mom got the book new or used. Even so, that does not answer the question. Regardless, you’ll probably never know the answer.

Its a normal sign of doubt.
Doubt in your mothers trust.
Have a private talk with her about how you see her attentions with the other man , and tell her how it makes YOU FEEL.
( be serious) and do not let her laugh it off
If she laughs it off? Ask IF its Ok and nothing then she wouldn’t mind you talking with your father ( her husband)about it at the Dinner table?