Assuming you don't accept returns -
Dear buyer,,
I received the querie regarding your recent purchase.
Just to clarify what was listed. The listing was for 2 Books as mentioned in the title and shown in the pictures.
I am sorry if you thought it was for 3 books as it was clearly shown to be for 2 books.
Whilst I am happy to accept returns for items seriously damaged in the post, unfortunately I don't accept change of mind returns especially as some buyers in the past have photocopied recipes and wanting a return.
I accepted your offer of a discounted price on these items and I will make an exception on my returns policy that if you post the items back, I will refund on this occasion.
Kind regards,
Then if the buyer returns, refund the item price only.
I wouldn't be too concerned about a possible neg as the buyer will undoubtedly mention the refund in the feedback which will make it against policy and eBay will remove it.
I like to ask buyers like this to help me understand what went wrong with my listing so that I can improve them, eg I'll say that I thought I had communicated all of the details clearly (and include where those details were) so would really appreciate their input on what caused the confusion or advice on what I missed to help make sure something like this doesn't happen again.
It's a fairly passive tactic, but if they're trying it on, they know full well the listing was clear, and I generally don't hear from those ones again. If they made an error, it hopefully prompts them to look at the listing again and realise the mistake was theirs - it's amazing how quickly a buyer's attitude can change from angry and demanding, to apologetic etc, and the best thing is, it's a relatively understanding and empathetic reply, so the buyer doesn't feel confronted and defensive, and you don't have to come right out and say "what flipping listing were you looking at?" 
