I am very familiar with the book in question and the company.
I also know that you actually requested the book, whether you know it or not.
You requested a "freebie" and at that time agreed to further books. It was in the "fine print" on the card you mailed in for your freebie.
They WILL put this on your credit report, if it isn't there already. It depends on if it has moved from inside collections to third party collections yet.
The best thing to do is to say you never got the book, and then demand that they prove that you did, unless you already talked to them and let them know you have it.
If you already blabbed to them that you did get it, you will have no choice but to return it, at your expense.
Always beware of "freebies", as they are NEVER "free".
This situation does not fall in to the "unsoliced merchandise" rule previously stated. You did request the book, although you were not aware that you did.