My Dear Old Friend,
If you are reading this note, then you remember your oath. I am pleased. I understand that a certain book is missing and you seek it. I would very much like to read this book. Meeting directly is impossible. Leave the book hidden in a location of your choice and pass that information to the tea vendor. We shall retrieve the book and make sure it is returned to the rightful owner, for a small "finder’s fee."
Do not fail. I would hate to lose such a good friend…