The reason you are getting the "Unable to retrieve asin B00RXX81JY from Amazon" is that the book pointed to by B00RXX81JY (Finders Keepers Kindle Edition by Stephen King) is not accessible through the Product Advertising API, which is what MyBookTable uses to retrieve book information from Amazon. It is not clear why Amazon has disabled access to this book through the API, though it may be because the title is not currently available for purchase on their website. To be clear, this is not a bug in MyBookTable or a problem with your website, this is simply caused by Amazon deliberately turning off data retrieval for this book. We are working on making the error messages in our plugin more descriptive in general, and this particular message will be improved in the next plugin update, which should prevent further confusion