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You would make a post in the book bazaar saying that you are offering a 2-1 deal on your bookshelf. Then, ask the responders to order one book and either PM you or post in the thread (your choice--some members prefer one, some the other) which book they would like free. You would then remove the free book from you bookshelf and send it with the one they ordered.
I've also heard that some people will just have the member order both books and will send one of the credits back. But that can run you in to trouble if you get a lot of people responding to your deal since the site limits you to the 50 credits monthly that you can transfer between members. If you like to use your credits in games or RAOK, etc. that may limit the number of credits you have available to send back to people.